Booster circuit, semiconductor device, and electronic apparatus

ABSTRACT

A conventional circuit requires a booster circuit for generating a voltage higher than an external power supply voltage, thus low power consumption is difficult to be achieved. In addition, a display device incorporating the aforementioned conventional switching element for booster circuit has problems in that the current load is increased and the power supply becomes unstable with a higher output current. The invention provides a booster circuit including a first transistor, a second transistor, a first capacitor element, a second capacitor element, a diode, and an inverter, wherein one electrode of the first transistor is maintained at a predetermined potential, the output of the inverter is connected to the gate electrode of the first transistor and one electrode of the second transistor through the second capacitor element, the input of the inverter is connected to the other electrode of the first transistor through the first capacitor element and connected to the gate electrode of the second transistor, and the diode is connected between the other electrode of the first transistor and the other electrode of the second transistor so as to be forwardly biased.

This application is a continuation of U.S. application Ser. No.11/829,319, filed on Jul. 27, 2007 now U.S. Pat. No. 7,432,757 which isa divisional of U.S. application Ser. No. 11/074,128, filed on Mar. 7,2005 (now U.S. Pat. No. 7,256,642 issued Aug. 14, 2007).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a booster circuit having a newconfiguration, and more particularly, the invention relates to asemiconductor device using a charge pump as a booster circuit. Also, theinvention relates to an electronic apparatus having the semiconductordevice.

2. Description of the Related Art

The booster circuit is classified into the one with a coil and the onewith a capacitor element. The latter one with a capacitor element isgenerally called a charge pump. A conventional charge pump comprises: afirst boosting block including a booster circuit for generating avoltage higher than an external power supply voltage, a diode that isconnected to an output of the booster circuit, maintains the outputvoltage of the booster circuit at a predetermined level, and has a zenervoltage higher than the external power supply voltage, and a voltagedividing resistor element for generating a reference voltage of apredetermined level in accordance with the voltage maintained at apredetermined level by the diode; and a second boosting block thatgenerates and outputs a voltage of which the output level is controlledto be a predetermined level in accordance with the reference voltagegenerated by the first boosting block, has a higher output currentcapacity than the first boosting block, and does not operate in astandby mode (see Patent Document 1).

[Patent Document 1] Japanese Patent Laid-Open No. 7-79561

In the circuit disclosed in Patent Document 1, which requires a boostercircuit for generating a voltage higher than an external power supplyvoltage, low power consumption is difficult to be achieved.

In addition, a display device incorporating the aforementionedconventional switching element for charge pump has the followingproblem. The charge pump, unlike other switching regulators, generallydoes not have a function to feed back an output voltage to stabilize theoutput, leading to a heavier current load and an unstable power supplywith a higher output current.

SUMMARY OF THE INVENTION

The invention provides a charge pump having a different configurationthan the one disclosed in Patent Document 1, and a semiconductor deviceusing the charge pump.

In view of the foregoing, the invention provides a booster circuithaving the following configurations.

A booster circuit of the invention is characterized by comprising afirst transistor, a second transistor, a first capacitor element, asecond capacitor element, a diode, and an inverter, wherein oneelectrode of the first transistor is maintained at a predeterminedpotential, the output of the inverter is connected to the gate electrodeof the first transistor and one electrode (first electrode) of thesecond transistor through the second capacitor element, the input of theinverter is connected to the other electrode of the first transistorthrough the first capacitor element and connected to the gate electrodeof the second transistor, and the diode is connected between the otherelectrode (second electrode) of the first transistor and the otherelectrode of the second transistor so as to be forwardly biased.

A booster circuit of the invention having another configuration ischaracterized by comprising a first transistor, a second transistor, afirst capacitor element, a second capacitor element, a diode, and aninverter, wherein one electrode of the first transistor is maintained ata predetermined potential, the output of the inverter is connected tothe gate electrode of the first transistor and one electrode of thesecond transistor through the second capacitor element, the input of theinverter is connected to the other electrode of the first transistorthrough the first capacitor element and connected to the gate electrodeof the second transistor, and the diode is connected to the otherelectrode of the first transistor so as to be forwardly biased.

The booster circuit having the aforementioned configuration ischaracterized in that the first transistor and the second transistorhave N-type conductivity and the predetermined potential is a high levelpotential, or the first transistor and the second transistor have P-typeconductivity and the predetermined potential is a low level potential.

A booster circuit of the invention having another configuration ischaracterized by comprising a first transistor, a second transistor, athird transistor, a capacitor element, a diode, and an inverter, whereinone electrode of the first transistor and the gate electrode of thesecond transistor are maintained at a predetermined potential, theoutput of the inverter is connected to the gate electrode of the thirdtransistor, one electrode of the third transistor is connected to thegate electrode of the first transistor and one electrode of the secondtransistor, the other electrode of the first transistor is connected tothe other electrode of the second transistor, and the diode is connectedto the other electrode of the first transistor so as to be forwardlybiased.

The booster circuit having the aforementioned configuration ischaracterized in that the first transistor and the second transistorhave P-type conductivity whereas the third transistor has N-typeconductivity and the predetermined potential is a high level potential,or the first transistor and the second transistor have N-typeconductivity whereas the third transistor has P-type conductivity andthe predetermined potential is a low level potential.

A booster circuit of the invention having another configuration ischaracterized by comprising a first transistor, a second transistor, afirst capacitor element, a second capacitor element, a diode, and aninverter, wherein one electrode of the first transistor is maintained ata predetermined potential, the output of the inverter is connected tothe gate electrode of the second transistor and the other electrode ofthe first transistor through the second capacitor element, the input ofthe inverter is connected to the gate electrode of the first transistorand one electrode of the second transistor through the first capacitorelement, and the diode is connected to the gate electrode of the firsttransistor so as to be forwardly biased.

A booster circuit of the invention having another configuration ischaracterized by comprising a first transistor, a second transistor, afirst capacitor element, a second capacitor element, a diode, and aninverter, wherein the diode is connected to the first capacitor elementand one electrode of the first transistor so as to be forwardly biasedand maintained at a predetermined potential, the output of the inverteris connected to the gate electrode of the second transistor and theother electrode of the first transistor through the second capacitorelement, and the input of the inverter is connected to the one electrodeof the first transistor through the gate electrode of the firsttransistor and the first capacitor element.

A booster circuit of the invention having another configuration ischaracterized by comprising a first transistor, a second transistor, afirst capacitor element, a second capacitor element, a diode, and aninverter, wherein the diode is connected to the first capacitor elementand one electrode of the second transistor and maintained at apredetermined potential, the output of the inverter is connected to thegate electrode of the second transistor and one electrode of thetransistor through the second capacitor element, the input of theinverter is connected to the gate electrode of the first transistor andconnected to the one electrode of the second transistor through thefirst capacitor element.

A booster circuit of the invention having another configuration ischaracterized by comprising a first transistor, a second transistor, athird transistor, a first capacitor element, a second capacitor element,and a diode, wherein the diode is connected to the first capacitorelement, the gate electrode of the first transistor and one electrode ofthe second transistor and maintained at a predetermined potential, thegate electrode of the first transistor and the one electrode of thesecond transistor are connected to the gate electrode of the thirdtransistor through the first capacitor element, one electrode of thefirst transistor is connected to the other electrode of the secondtransistor, and the other electrode of the first transistor is connectedto one electrode of the third transistor.

The booster circuit having the aforementioned configuration ischaracterized in that a clock signal is inputted to the gate electrodeof the third transistor.

The booster circuit having the aforementioned configuration ischaracterized in that the first transistor has N-type conductivitywhereas the second transistor has P-type conductivity and thepredetermined potential is a high level potential, or the firsttransistor has P-type conductivity whereas the second transistor hasN-type conductivity and the predetermined potential is a low levelpotential.

A booster circuit of the invention having another configuration ischaracterized by comprising a first transistor, a second transistor, athird transistor, a fourth transistor, a first capacitor element, asecond capacitor element, a third capacitor element, and an inverter,wherein one electrode of the first transistor is maintained at apredetermined potential and connected to one electrode of the thirdtransistor, the output of the inverter is connected to the gateelectrode of the second transistor and connected to the gate electrodeof the third transistor and one electrode of the fourth transistorthrough the first capacitor element, the input of the inverter isconnected to the gate electrode of the first transistor and oneelectrode of the second transistor through the second capacitor elementand connected to the gate electrode of the fourth transistor and theother electrode of the third transistor through the third capacitorelement, the other electrode of the first transistor is connected to thegate electrode of the third transistor and the one electrode of thefourth transistor, and the other electrode of the second transistor isconnected to the other electrode of the fourth transistor.

The booster circuit having the aforementioned configuration ischaracterized in that the first, second and third transistors haveN-type conductivity whereas the fourth transistor has P-typeconductivity and the predetermined potential is a high level potential,or the first, second and third transistors have P-type conductivitywhereas the fourth transistor has N-type conductivity and thepredetermined potential is a low level potential.

The booster circuit having the aforementioned configuration ischaracterized in that a clock signal is inputted to the inverter.

In the aforementioned booster circuit of the invention, a thin filmtransistor (hereinafter also referred to as a TFT) can be used as atransistor.

According to the invention, a booster circuit having a new configurationcan be provided. As a result, low power consumption, high output currentand high output potential can be achieved.

Since the charge pump of the invention can be constituted by thin filmtransistors, it can be formed integrally with a pixel portion of aliquid crystal display device, a display device having light emittingelements (hereinafter also referred to as a light emitting device), andother display devices. Accordingly, the clock frequency of a switchingelement using the charge pump can be selected depending on a displaymode, resulting in lower power consumption of the display device.Further, the integral formation allows the external circuit to besimplified. Thus, the number of components of the circuit can be reducedand reduction in cost can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are circuit diagrams each showing a charge pump of theinvention.

FIG. 2 is a circuit diagram showing a charge pump of the invention.

FIG. 3 is a circuit diagram showing a charge pump of the invention.

FIGS. 4A and 4B are circuit diagrams each showing a charge pump of theinvention.

FIG. 5 is a circuit diagram showing a charge pump of the invention.

FIG. 6 is a circuit diagram showing a charge pump of the invention.

FIGS. 7A and 7B are circuit diagrams each showing a charge pump of theinvention.

FIG. 8 is a circuit diagram showing a charge pump of the invention.

FIG. 9 is a circuit diagram showing a charge pump of the invention.

FIG. 10 is a circuit diagram showing a charge pump of the invention.

FIG. 11 is a circuit diagram showing a charge pump of the invention.

FIG. 12 is a diagram showing a display device having a charge pump ofthe invention.

FIG. 13 is a circuit diagram showing a charge pump.

FIG. 14 is a circuit diagram showing a charge pump.

FIGS. 15A to 15H are views each showing an electronic apparatus having acharge pump of the invention.

FIG. 16 is a diagram showing a regulator having a charge pump of theinvention.

FIGS. 17A and 17B are diagrams showing a regulator having a charge pumpof the invention.

FIG. 18 is a top plan view showing a charge pump of the invention.

FIG. 19 is a cross sectional view of a charge pump and a pixel portionof the invention.

FIG. 20 is a circuit diagram showing a charge pump of the invention.

FIG. 21 is a circuit diagram showing a charge pump of the invention.

FIG. 22 is a circuit diagram showing a charge pump of the invention.

FIG. 23 is a circuit diagram showing a charge pump of the invention.

FIG. 24 is a circuit diagram showing a charge pump of the invention.

FIG. 25 is a circuit diagram showing a charge pump of the invention.

FIG. 26 is a circuit diagram showing a charge pump of the invention.

FIG. 27 is a circuit diagram showing a charge pump of the invention.

FIG. 28 is a circuit diagram showing a charge pump of the invention.

FIG. 29 is a circuit diagram showing a charge pump of the invention.

FIG. 30 is a circuit diagram showing a charge pump of the invention.

FIG. 31 is a circuit diagram showing a charge pump of the invention.

FIG. 32 is a circuit diagram showing a charge pump of the invention.

FIG. 33 is a circuit diagram showing a charge pump of the invention.

FIG. 34 is a circuit diagram showing a charge pump of the invention.

FIG. 35 is a circuit diagram showing a charge pump of the invention.

FIG. 36 is a circuit diagram showing a charge pump of the invention.

FIG. 37 is a circuit diagram showing a charge pump of the invention.

FIG. 38 is a circuit diagram showing a charge pump of the invention.

FIG. 39 is a circuit diagram showing a charge pump of the invention.

FIG. 40 is a circuit diagram showing a charge pump of the invention.

FIG. 41 is a circuit diagram showing a charge pump of the invention.

FIG. 42 is a circuit diagram showing a charge pump of the invention.

FIG. 43 is a circuit diagram showing a charge pump of the invention.

FIG. 44 is a circuit diagram showing a charge pump of the invention.

FIG. 45 is a circuit diagram showing a charge pump of the invention.

FIG. 46 is a circuit diagram showing a charge pump of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Although the invention will be described by way of Embodiment Modes withreference to the accompanying drawings, it is to be understood thatvarious changes and modifications will be apparent to those skilled inthe art. Therefore, unless such changes and modifications depart fromthe scope of the invention, they should be construed as being includedtherein.

The identical portions or portions having the same function are denotedby the same reference numerals in all the drawings for describingEmbodiment Modes, and will be described in no more detail.

Although a transistor has three terminals of gate, source and drainterminals, there is no clear structural distinction between the sourceelectrode terminal (source electrode) and the drain electrode terminal(drain electrode). Therefore, in the description of the connectionbetween elements, one of the source electrode and the drain electrode isreferred to as one electrode while the other thereof is referred to asthe other electrode.

Embodiment Mode 1

In this embodiment mode, configuration and operation of the charge pumpare described. Note that a plural-stage charge pump can multiply avoltage. In a four-stage Dickson charge pump as shown in FIG. 13, anoutput voltage can be ideally boosted up to four times. Described inthis embodiment mode is a circuit configuration that can be used for thefirst stage of the circuit.

A charge pump shown in FIG. 1A includes a first transistor 101, a secondtransistor 102, a first capacitor element 103, a second capacitorelement 104, an inverter 105, and a diode 106. The capacitor element 103in FIG. 1A corresponds to a capacitor element C1 in FIG. 13. The diode106 has a function corresponding to a diode D2 in FIG. 13. The firsttransistor 101, the second transistor 102 and the capacitor element 104collectively function as a diode D1 in FIG. 13.

It is assumed that a high level potential is Vdd and a low levelpotential is 0 V for simplicity, though the invention is not limited tothis. Accordingly, Vdd and 0 V are inputted to the inverter 105 as ahigh signal and a low signal, respectively. Also, Vdd and 0 V areoutputted from the inverter 105 as a high signal and a low signal,respectively. In this embodiment mode, the first transistor 101 and thesecond transistor 102 have N-type conductivity. The diode 106 may be anyone of a PN diode, a PIN diode, a Schottky diode, and a diode connectedtransistor. In the case of a diode connected transistor being used, itmay have either N-type conductivity or P-type conductivity. The diode106 may also have any element configuration and circuit configuration.For example, circuit configurations described in Embodiment Modes 4 to 7below may be adopted for the diode 106. The first transistor 101, thesecond transistor 102 and the capacitor element 104 collectivelyfunction as a diode D1 in FIG. 13.

The connection between each element is described now.

One electrode of the first transistor 101 is connected to a power supplyto be maintained at a high level potential of Vdd. The output of theinverter 105 (point S) is connected to the gate electrode of the firsttransistor 101 and one electrode of the second transistor 102 (point R)through the second capacitor element 104. The input of the inverter 105(point Q) is connected to the other electrode of the first transistor101 and the input of the diode 106 (point P) through the first capacitorelement 103 and connected to the gate electrode of the second transistor102. The other electrode of the second transistor 102 is connected tothe output of the diode 106. That is, the diode 106 is connected betweenthe other electrode of the first transistor 101 and the other electrodeof the second transistor 102 so as to be forwardly biased.

The operation of the charge pump having such a circuit configuration isdescribed below.

A clock signal with a high signal of Vdd and a low signal of 0 V isinputted to the input of the inverter 105 (point Q). If a low signal isinputted to the input of the inverter 105 (point Q), for example, a highsignal is inputted to the second capacitor element 104 while a lowsignal is inputted to the gate electrode of the second transistor 102and the first capacitor element 103. At this time, the other electrodeof the first transistor 101 (point P) is at 0 V and the gate electrodethereof is at Vdd. Thus, the first transistor 101 of which the gateelectrode is applied with a high voltage is turned on. Since the firsttransistor 101 is turned on, the potential at the point P rises, therebya predetermined charge is accumulated in the first capacitor element103. The second transistor 102 of which the gate electrode is at 0 V isturned off. Accordingly, the voltage at both ends of the secondcapacitor element 104 can be held.

When the next clock waveform, namely a high signal is inputted to thepoint Q, a high signal is inputted to the gate electrode of the secondtransistor 102 and the first capacitor element 103, while a low signalis inputted to the second capacitor element 104. Since a high signal isinputted to the first capacitor element 103, the voltage at the point Pincreases by Vdd corresponding to a high signal, leading to increase inVout across the diode 106. The second transistor 102 of which the gateelectrode is at Vdd is turned on. Thus, current flows from Vout to thepoint R. When the voltage between the point Q and the point R becomesequal to the threshold voltage (Vth) of the second transistor 102, thesecond transistor 102 is turned off. Accordingly, the voltage at thepoint R is lower than that at the point Q by Vth. In other words, thefirst transistor 101 of which the gate electrode is applied with a lowpotential is turned off, thus the charge accumulated in the firstcapacitor element 103 does not leak through the first transistor 101 andcan be outputted to Vout certainly. As a result, the potential at theinput of the diode 106 (point P) becomes higher than that at the outputthereof (Vout), and a predetermined current can be outputted to Vout,thereby Vout is boosted. As the point S is at 0 V at this time, thevoltage at both ends of the second capacitor element 104 is equal toVdd−Vth.

When the next clock waveform, namely a low signal is inputted to thepoint Q, a high signal is inputted to the second capacitor element 104while a low signal is inputted to the gate electrode of the secondtransistor 102 and the first capacitor element 103. As set forth above,the predetermined charge has already been accumulated in the secondcapacitor element 104, and the voltage at the point R is equal toVdd−Vth. The point R is further added with Vdd corresponding to a highsignal, thus the potential at the gate electrode of the first transistor101 rises. Since the second transistor 102 is off at this time, thecharge accumulated in the second capacitor element 104 is held and thepotential at the point R rises by Vdd. Accordingly, the first transistor101 is turned on as described above. The gate electrode of the firsttransistor 101 has a voltage higher than Vdd+Vth at this time,therefore, the potential at the point P is equal to Vdd. It is fearedthat the potential at the input of the diode 106 (point P) may be lowerthan that at the output thereof, however, no current flows in view ofthe characteristics of the diode. Thus, Vout can be maintained at a highlevel.

When the next clock wave form, namely a high signal is inputted to thepoint Q, as set forth above, a high signal is inputted to the gateelectrode of the second transistor 102 and the first capacitor element103, while a low signal is inputted to the second capacitor element 104.Then, a high signal is inputted to the first capacitor element 103,thereby the voltage at the point P increases by Vdd corresponding to ahigh signal, leading to increase in Vout across the diode 106. Thesecond transistor 102 of which the gate electrode is at Vdd is turnedon. Thus, current flows from Vout to the point R. When the voltagebetween the point Q and the point R becomes equal to the thresholdvoltage (Vth) of the second transistor 102, the second transistor 102 isturned off. Accordingly, the voltage at the point R is lower than thatat the point Q by Vth. In other words, the first transistor 101 of whichthe gate electrode is applied with a low potential is turned off, thusthe charge accumulated in the first capacitor element 103 does not leakthrough the first transistor 101 and can be outputted to Vout certainly.As a result, the potential at the input of the diode 106 (point P)becomes higher than that at the output thereof (Vout), and apredetermined current can be outputted to Vout, thereby Vout is boosted.As the point S is at 0 V at this time, the voltage at both ends of thesecond capacitor element 104 is equal to Vdd−Vth.

By repeating such operation, the potential at Vout can be increased to2×Vdd (see FIG. 1B).

Note that the potential at Vout can be increased to 2×Vdd only when noload is connected to Vout. If a load (resistor, capacitor, transistor,circuit, or the like) is provided, which consumes current, the potentialat Vout becomes lower than 2×Vdd.

In the charge pump according to this embodiment mode, the voltage at thegate electrode of the first transistor 101 can be made higher thanVdd+Vth by the second capacitor element 104. In other words, voltagedrop due to the threshold voltage of the first transistor 101 can beprevented, namely, it can be prevented that the potential at Vout dropsby Vth of the first transistor 101. When a high signal is inputted tothe point Q, the voltage at the point R becomes equal to Vdd−Vth throughthe second transistor 102. At this time, charge leak can be prevented byturning the first transistor 101 off.

This embodiment mode is not limited to the connection shown in FIG. 1A.For example, the point S and the point Q are connected to each otherthrough the inverter 105, though the invention is not limited to this.

Different signals may be supplied to the point Q and the point S insteadof providing the inverter 105. In this case, inverted signals aredesirably supplied to the point Q and the point S, though the inventionis not limited to this. The signals supplied to the point Q and thepoint S are not necessarily inverted as long as the circuit operatesnormally.

A high signal inputted to the point Q is not necessarily equal to Vdd,and may have a voltage lower or higher than Vdd. Similarly, a low signalinputted to the point Q is not necessarily equal to 0 V, and may have avoltage lower or higher than 0 V.

A high signal inputted to the point S is not necessarily equal to Vdd,and may have a voltage lower or higher than Vdd. Similarly, a low signalinputted to the point S is not necessarily equal to 0 V, and may have avoltage lower or higher than 0 V.

Similarly, a high signal inputted to the point S and a high signalinputted to the point Q may have different potentials. Similarly, a lowsignal inputted to the point S and a low signal inputted to the point Qmay have different potentials.

In the aforementioned charge pump, thin film transistors can be used asthe transistors. As a result, the charge pump can be formed integrallywith a display device or a nonvolatile memory such as a flash memory.When thin film transistors are used in the charge pump, however, it isdifficult to raise the potential to a predetermined level because of ahigh threshold voltage. In addition, variations in threshold voltages ofthin film transistors may cause variations in potentials to beoutputted. Meanwhile, when the charge pump according to this embodimentmode is used, the second capacitor element 104 prevents voltage drop dueto threshold voltage as described above. Therefore, the charge pumpaccording to this embodiment mode is extremely effective in the case ofusing thin film transistors having a threshold voltage higher thantransistors formed over a silicon wafer.

The charge pump including thin film transistors can be formed integrallywith a liquid crystal display device, a light emitting device and otherdisplay devices.

In such a case, either or both of the first capacitor element 103 andthe second capacitor element 104 may be formed integrally with thedisplay device. The integral formation with the display device allowsreduction in the number of components. On the other hand, if thecapacitor element is not formed integrally with the display device, thecapacitance of the capacitor element can be increased. The firstcapacitor element 103 is required to have higher capacitance than thesecond capacitor element 104. Thus, the smaller second capacitor element104 may be formed integrally with the display device, thereby the numberof components is reduced and cost reduction is achieved. Meanwhile, thelarger first capacitor element 103 may be formed separately from thedisplay device, thereby the capacitance of the first capacitor element103 can be increased.

Although the first transistor 101 and the second transistor 102 haveN-type conductivity in this embodiment mode, the conductivity of thetransistors is not exclusively limited. For example, a circuitconfiguration where the first transistor 101 and the second transistor102 have P-type conductivity and one electrode of the first transistor101 is maintained at a low level potential (0 V in this embodiment mode)may be adopted as well. In this case, the direction of the diode 106shown in FIG. 1A is desirably reversed. That is, in this embodimentmode, the conductivity of the transistor can be changed depending onwhether one electrode of the first transistor 101 is maintained at ahigh level potential or a low level potential.

Embodiment Mode 2

Described in this embodiment mode are configuration and operation of thecharge pump, which are different from those shown in Embodiment Mode 1.In this embodiment, a circuit configuration that can be used for thefirst stage is described as is in Embodiment Mode 1.

A charge pump shown in FIG. 2 includes, similarly to that shown in FIG.1A, the first transistor 101, the second transistor 102, the firstcapacitor element 103, the second capacitor element 104, the inverter105, and the diode 106. The first capacitor element 103 corresponds tothe capacitor element C1 in FIG. 13. The diode 106 has a functioncorresponding to the diode D2 in FIG. 13. It is assumed that a highlevel potential is Vdd while a low level potential is 0 V forsimplicity, though the invention is not limited to this. Accordingly,Vdd is outputted from the inverter 105 as a high signal while 0 V isoutputted from the inverter 105 as a low signal. In this embodimentmode, the first transistor 101 and the second transistor 102 have N-typeconductivity. The diode 106 may be any one of a PN diode, a PIN diode, aSchottky diode, and a diode connected transistor. The diode 106 may alsohave any element configuration and circuit configuration. For example,circuit configurations described in Embodiment Modes 4 to 7 may beadopted for the diode 106. The first transistor 101, the secondtransistor 102 and the capacitor element 104 collectively function asthe diode D1 in FIG. 13.

The connection between each element is described now. The connection ofthe charge pump in FIG. 2 is similar to that shown in FIG. 1A exceptthat the other electrode of the second transistor 102 is connected tothe input of the diode 106 (point P).

The operation of the charge pump having such a circuit configuration issimilar to that described in Embodiment Mode 1.

Similarly to Embodiment Mode 1, a voltage of 2×Vdd can be outputted toVout by repeating the operation (see FIG. 1B).

In the charge pump according to this embodiment mode, voltage drop dueto the threshold voltage of the first transistor 101 can be prevented bythe second capacitor element 104. When a high signal is inputted to thepoint Q, the voltage at the point R becomes equal to Vdd−Vth through thesecond transistor 102. At this time, charge leak can be prevented byturning the first transistor 101 off.

As described in Embodiment Mode 1, this embodiment mode is not limitedto the connection shown in FIG. 2.

In the aforementioned charge pump, thin film transistors can be used asthe transistors. As a result, the charge pump can be formed integrallywith a display device or a nonvolatile memory such as a flash memory.When thin film transistors are used in the charge pump, however, it isdifficult to raise the potential to a predetermined level because of ahigh threshold voltage. In addition, variations in threshold voltages ofthin film transistors may cause variations in potentials to beoutputted. Meanwhile, when the charge pump according to this embodimentmode is used, the second capacitor element 104 prevents voltage drop dueto threshold voltage as described above. Therefore, the charge pumpaccording to this embodiment mode is extremely effective in the case ofusing thin film transistors having a threshold voltage higher thantransistors formed on a silicon wafer.

The charge pump including thin film transistors can be formed integrallywith a liquid crystal display device, a light emitting device and otherdisplay devices.

Although the first transistor 101 and the second transistor 102 haveN-type conductivity in this embodiment mode, the conductivity of thetransistors is not exclusively limited. For example, a circuitconfiguration where the first transistor 101 and the second transistor102 have P-type conductivity and one electrode of the first transistor101 is maintained at a low level potential (0 V in this embodiment mode)may be adopted as well. In this case, the direction of the diode 106 isdesirably reversed to that shown in FIG. 2. That is, in this embodimentmode, the conductivity of the transistor can be changed depending onwhether one electrode of the first transistor 101 is maintained at ahigh level potential or a low level potential.

Embodiment Mode 3

Described in this embodiment mode are configuration and operation of thecharge pump, which are different from those shown in Embodiment Modes 1and 2. In this embodiment mode, a circuit configuration that can be usedfor the first stage is described as is in Embodiment Modes 1 and 2.

A charge pump shown in FIG. 3 includes a first transistor 111, a secondtransistor 112, a third transistor 113, the capacitor element 103, theinverter 105, and the diode 106. The capacitor element 103 in FIG. 3corresponds to the capacitor element C1 in FIG. 13. The diode 106 has afunction corresponding to the diode D2 in FIG. 13. It is assumed that ahigh level potential is Vdd while a low level potential is 0 V forsimplicity, though the invention is not limited to this. Accordingly,Vdd is outputted from the inverter 105 as a high signal while 0 V isoutputted from the inverter 105 as a low signal. In this embodimentmode, the first transistor 111 and the second transistor 112 have P-typeconductivity and the third transistor 113 has N-type conductivity. Thediode 106 may be any one of a PN diode, a PIN diode, a Schottky diode,and a diode connected transistor. The diode 106 may also have anyelement configuration and circuit configuration. For example, circuitconfigurations described in Embodiment Modes 4 to 7 may be adopted forthe diode 106. The first transistor 111, the second transistor 112 andthe third transistor 113 collectively function as the diode D1 in FIG.13.

The connection between each element is described below.

One electrode of the first transistor 111 and the gate electrode of thesecond transistor 112 are connected to a power supply to be maintainedat a high level potential of Vdd. The output of the inverter 105 isconnected to the gate electrode of the third transistor 113. Oneelectrode of the third transistor 113 is connected to the gate electrodeof the first transistor 111 and one electrode of the second transistor112 (point R). The other electrode of the first transistor 111 isconnected to the other electrode of the second transistor 112. The inputof the inverter 105 (point Q) is connected to the input of the diode 106(point P) through the capacitor element 103. That is, the diode 106 isconnected to the other electrode of the first transistor 111 so as to beforwardly biased.

The operation of the charge pump having such a circuit configuration isdescribed.

A clock signal with a high signal of Vdd and a low signal of 0 V isinputted to the input of the inverter 105 (point Q). If a low signal isinputted to the input of the inverter 105 (point Q), for example, a highsignal is inputted to the third transistor 113 while a low signal isinputted to the capacitor element 103. At this time, the thirdtransistor 113 of which the other electrode (drain electrode) is at 0 Vand the gate electrode is at Vdd is turned on. Thus, since the gateelectrode of the first transistor 111 becomes 0 V and one electrodethereof is at Vdd, the first transistor 111 is turned on. Accordingly,the potential at the point P becomes Vdd, thereby a predetermined chargeis accumulated in the capacitor element 103. The second transistor 112of which one electrode (source electrode, point P) is at Vdd and thegate electrode is at Vdd is turned off at this time.

When the next clock waveform, namely a high signal is inputted to thepoint Q, a low signal is inputted to the third transistor 113 while ahigh signal is inputted to the capacitor element 103. Since a highsignal is inputted to the capacitor element 103, Vdd corresponding to ahigh signal is added to the charge that has already been accumulated inthe capacity element 103 and the voltage at the point P increases,leading to increase in Vout across the diode 106. The third transistor113 of which the other electrode (source electrode, point R) is at 0 Vand the gate electrode is at 0 V is turned off at this time. The otherelectrode of the second transistor 112 (source electrode, point P)becomes equal to the potential at one end of the capacitor element 103,namely 2×Vdd, and the gate electrode thereof is at Vdd, thus the secondtransistor 112 is turned on. The potential at the point R rises to thepotential at the point P. Then, the potential at the gate electrode ofthe first transistor 111 (point R) becomes equal to that at the sourceelectrode thereof (point P), thereby the first transistor 111 is turnedoff. As a result, the potential at the input of the diode 106 (point P)becomes higher than that at the output thereof, thus a predeterminedvoltage can be outputted to Vout. Since the first transistor 111 is offat this time, the charge accumulated in the capacitor element 103 doesnot flow to the first transistor 111 and can be outputted to Voutcertainly.

The potential at Vout can be increased to 2×Vdd by repeating suchoperation.

The charge pump according to this embodiment mode is advantageous inthat the voltage of 2×Vdd held in the capacitor element 103 is not lostsince the second transistor 112 is on while the first transistor 111 isoff when a predetermined voltage is outputted to Vout, namely when ahigh signal is inputted to the point Q. In addition, the charge pumpaccording to this embodiment mode can make the point P be at the voltageof Vdd, since the voltage at the point R becomes 0 V and the firsttransistor 111 is turned on when a low signal is inputted to the pointQ. That is, the voltage at the point P does not become Vdd−Vth.Therefore, a predetermined voltage can be outputted to Voutindependently of the threshold voltage (Vth) of the first transistor111. In other words, a predetermined charge can be accumulated withoutbeing affected by voltage drop due to the threshold voltage of the firsttransistor 111.

This embodiment mode is not limited to the connection shown in FIG. 3.For example, the point S may be connected to the point Q. Further,although the point Q and the gate electrode of the third transistor 113are connected to each other though the inverter 105, the invention isnot limited to this.

Different signals may be supplied to the point Q and the gate electrodeof the third transistor 113 instead of providing the inverter 105. Inthis case, inverted signals are desirably supplied to the point Q andthe gate electrode of the third transistor 113, though the invention isnot limited to this. The signals supplied to the point Q and the gateelectrode of the third transistor 113 are not necessarily inverted aslong as the circuit operates normally.

A high signal inputted to the point Q is not necessarily equal to Vdd,and may have a voltage lower or higher than Vdd. Similarly, a low signalinputted to the point Q is not necessarily equal to 0 V, and may have avoltage lower or higher than 0 V.

A high signal inputted to the gate electrode of the third transistor 113is not necessarily equal to Vdd, and may have a voltage lower or higherthan Vdd. Similarly, a low signal inputted to the gate electrode of thethird transistor 113 is not necessarily equal to 0 V, and may have avoltage lower or higher than 0 V.

A high signal inputted to the gate electrode of the third transistor 113and a high signal inputted to the point Q may have different potentials.Similarly, a low signal inputted to the gate electrode of the thirdtransistor 113 and a low signal inputted to the point Q may havedifferent potentials.

A signal inputted to the point S does not necessarily have a voltage of0 V, and may have a voltage lower or higher than 0 V.

In the aforementioned charge pump, thin film transistors can be used asthe transistors. As a result, the charge pump can be formed integrallywith a display device or a nonvolatile memory such as a flash memory.When thin film transistors are used in the charge pump, however, it isdifficult to raise the potential to a predetermined level because of ahigh threshold voltage. In addition, variations in threshold voltages ofthin film transistors may cause variations in potentials to beoutputted. Meanwhile, when the charge pump according to this embodimentmode is used, a predetermined charge can be outputted without beingaffected by voltage drop due to threshold voltage, since the voltage atthe point P can be at Vdd when a low signal is inputted to the point Qas described above. Therefore, the charge pump according to thisembodiment mode is extremely effective in the case of using thin filmtransistors having a threshold voltage higher than transistors formed ona silicon wafer. The charge pump according to this embodiment mode isalso advantageous in that a charge does not leak through the firsttransistor 111 since the first transistor 111 can be turned off by thesecond transistor 112 when a high signal is inputted to the point Q.

The charge pump including thin film transistors can be formed integrallywith a liquid crystal display device, a light emitting device and otherdisplay devices.

Although the first transistor 111 and the second transistor 112 haveP-type conductivity and the third transistor 113 has N-type conductivityin this embodiment mode, the conductivity of the transistors is notexclusively limited. For example, a circuit configuration where thefirst transistor 111 and the second transistor 112 have N-typeconductivity, the third transistor 113 has P-type conductivity and oneelectrode of the first transistor 111 is maintained at a low levelpotential (0 V) may be adopted as well. In this case, the direction ofthe diode 106 is desirably reversed to that shown in FIG. 3 as shown inEmbodiment Mode 9 below. That is, in this embodiment mode, theconductivity of the transistor can be changed depending on whether oneelectrode of the first transistor 111 is maintained at a high levelpotential or a low level potential.

Embodiment Mode 4

Described in this embodiment mode are configuration and operation of thecharge pump, which are different from those shown in Embodiment Modes 1to 3. As set forth above, a plural-stage charge pump can multiply avoltage. In this embodiment mode, a circuit configuration that can beused for the second or later stage is described.

A charge pump shown in FIG. 4A includes a first transistor 121, a secondtransistor 122, the first capacitor element 103, a second capacitorelement 123, the inverter 105, and a diode 116. The first capacitorelement 103 in FIG. 4A corresponds to the capacitor element C1 in FIG.13. The diode 116 has a function corresponding to the diode D1 in FIG.13. The first transistor 121, the second transistor 122 and the secondcapacitor element 123 collectively function as the diode D2 in FIG. 13.

It is assumed that a high level potential is Vdd while a low levelpotential is 0 V for simplicity, though the invention is not limited tothis. Accordingly, Vdd is inputted to and outputted from the inverter105 as a high signal while 0 V is inputted to and outputted from theinverter 105 as a low signal. In this embodiment mode, the firsttransistor 121 has N-type conductivity and the second transistor 122 hasP-type conductivity. The diode 116 may be any one of a PN diode, a PINdiode, a Schottky diode, and a diode connected transistor. If the diodeconnected transistor is used, it may have either N-type conductivity orP-type conductivity. The diode 116 may also have any elementconfiguration and circuit configuration. For example, circuitconfigurations described in Embodiment Modes 1 to 3 may be adopted forthe diode 116.

The connection between each element is described below.

One electrode of the first transistor 121 and the input of the diode 116are connected to a power supply to be maintained at a high levelpotential of Vdd. The output of the inverter 105 is connected to thegate electrode of the second transistor 122 and the other electrode ofthe first transistor 121 (point R) through the second capacitor element123. The input of the inverter 105 (point Q) is connected to the gateelectrode of first transistor 121 and one electrode of the secondtransistor 122 through the first capacitor element 103. The output ofthe diode 116 (point P) is connected to the gate electrode of the firsttransistor 121. That is, the diode 116 is connected to the gateelectrode of the first transistor 121 so as to be forwardly biased.

The operation of the charge pump having such a circuit configuration isdescribed.

A clock signal with a high signal of Vdd and a low signal of 0 V isinputted to the input of the inverter 105 (point Q). For example, when alow signal is inputted to the input of the inverter 105 (point Q), ahigh signal is inputted to the second capacitor element 123 while a lowsignal is inputted to the first capacitor element 103. Then, the diode116 is turned on, thereby the voltage at the point P becomes Vdd.Further, the gate electrode of the first transistor 121 as well as oneelectrode (source electrode) thereof becomes Vdd, therefore, the firsttransistor 121 is turned off. That is, Vdd is outputted from the diode116 to the point P, thus one end of the first capacitor element 103becomes Vdd while the other end (point Q) to which a low signal isinputted becomes 0 V, thereby a charge corresponding to Vdd isaccumulated in the first capacitor element 103. Since the gate electrodeof the second transistor 122 has a high level potential at this time,the second transistor 122 is turned off.

When the next clock waveform, namely a high signal is inputted to thepoint Q, a low signal is inputted to the second capacitor element 123and a high signal is inputted to the first capacitor element 103. Atthis time, the voltage at the point P is 2×Vdd. Then, the firsttransistor 121 of which the gate electrode is at 2×Vdd is turned on,thereby the voltage at the gate electrode of the second transistor 122(point R) becomes Vdd. Since one electrode of the second transistor 122(point P) becomes 2×Vdd, the second transistor 122 is turned on. As aresult, a predetermined voltage can be outputted to Vout. A chargecorresponding to Vdd is accumulated in the second capacitor element 123since the voltage at the point R is Vdd and the inverter 105 outputs alow signal.

When the next clock waveform, namely a low signal is inputted to thepoint Q, a high signal is inputted to the second capacitor element 123while a low signal is inputted to the first capacitor element 103. Then,the diode 116 is turned on, a charge is supplied to the first capacitorelement 103, and the voltage at the point P becomes Vdd. That is, Vdd isoutputted from the diode 116, thereby one end of the first capacitorelement 103 becomes Vdd and the other end thereof (point Q) to which alow signal is inputted becomes 0 V. As a result, the voltage at thepoint P becomes Vdd and a charge corresponding to Vdd is accumulated inthe first capacitor element 103. Further, the gate electrode of thefirst transistor 121 as well as one electrode thereof (source electrode)becomes Vdd, thus the first transistor 121 is turned off. Accordingly,the charge in the second capacitor element 123 can be held. At thistime, the voltage at the gate electrode of the second transistor 122 is2×Vdd, thus the second transistor 122 is turned off. As a result, thecharge corresponding to Vout can be prevented from leaking to the pointP through the second transistor 122.

The potential at Vout can be increased to 2×Vdd by repeating suchoperation.

In the charge pump according to this embodiment mode, when a high signalis inputted to the point Q, the potential at the point R, namely thegate electrode of the second transistor 122 can be lowered (to Vdd)using the first transistor 121, thereby the potential at the point P canbe made equal to Vout. That is, the voltage at Vout does not become2×Vdd−Vth. Accordingly, a predetermined voltage can be outputted to Voutindependently of the threshold voltage (Vth) of the second transistor122. In other words, in the charge pump according to this embodimentmode, a predetermined voltage can be outputted to Vout without beingaffected by voltage drop due to the threshold voltage of the secondtransistor 122. Meanwhile, when a low signal is inputted to the point Q,the potential at the point R, namely the gate electrode of the secondtransistor 122 can be increased to 2×Vdd through the second capacitorelement 123. Therefore, it can be prevented that the potential at Voutis lowered due to current leak through the second transistor 122.

This embodiment mode is not limited to the connection shown in FIG. 4A.For example, the second capacitor element 123 is connected to the pointQ through the inverter 105, though this embodiment mode is not limitedto this.

Different signals may be supplied to the point Q and the capacitorelement 123 instead of providing the inverter 105. In this case,inverted signals are desirably supplied to the point Q and the capacitorelement 123, though the invention is not limited to this. The signalssupplied to the point Q and the capacitor element 123 are notnecessarily inverted as long as the circuit operates normally.

A high signal inputted to the point Q is not necessarily equal to Vdd,and may have a voltage lower or higher than Vdd. Similarly, a low signalinputted to the point Q is not necessarily equal to 0 V, and may have avoltage lower or higher than 0 V.

A high signal inputted to the capacitor element 123 is not necessarilyequal to Vdd, and may have a voltage lower or higher than Vdd.Similarly, a low signal inputted to the capacitor element 123 is notnecessarily equal to 0 V, and may have a voltage lower or higher than 0V.

Although the circuit in FIG. 4A is applied to the second stage in thisembodiment mode, it may be applied to the third or later stage as well.FIG. 4B shows an example of the circuit applied to the third stage. Adiode 150 shown in FIG. 4B corresponds to the diode D1 in FIG. 13whereas a capacitor element 153 corresponds to the capacitor element C1in FIG. 13.

Different signals may be supplied to the capacitor elements 153, 123 and103 instead of providing the inverter 105. In this case, invertedsignals are desirably supplied to the capacitor element 153 or 123 andthe capacitor element 103, though the invention is not limited to this.The signals supplied to the capacitor element 153 or 123 and thecapacitor element 103 are not necessarily inverted as long as thecircuit operates normally. Further, the same signal is desirablysupplied to the capacitor element 153 and the capacitor element 123,though the invention is not limited to this. Different timing or voltagesignals may be supplied to the capacitor element 153 and the capacitorelement 123 as long as the circuit operates normally.

A high signal inputted to the capacitor element 153 is not necessarilyequal to Vdd, and may have a voltage lower or higher than Vdd.Similarly, a low signal inputted to the capacitor element 153 is notnecessarily equal to 0 V, and may have a voltage lower or higher than 0V.

A high signal inputted to the capacitor element 123 is not necessarilyequal to Vdd, and may have a voltage lower or higher than Vdd.Similarly, a low signal inputted to the capacitor element 123 is notnecessarily equal to 0 V, and may have a voltage lower or higher than 0V.

A high signal inputted to the capacitor element 103 is not necessarilyequal to Vdd, and may have a voltage lower or higher than Vdd.Similarly, a low signal inputted to the capacitor element 103 is notnecessarily equal to 0 V, and may have a voltage lower or higher than 0V.

In the aforementioned charge pump, thin film transistors can be used asthe transistors. As a result, the charge pump can be formed integrallywith a display device or a nonvolatile memory such as a flash memory.When thin film transistors are used in the charge pump, however, it isdifficult to raise the potential to a predetermined level because of ahigh threshold voltage. In addition, variations in threshold voltages ofthin film transistors may cause variations in potentials to beoutputted. Meanwhile, when the charge pump according to this embodimentmode is used, a predetermined charge can be outputted without beingaffected by voltage drop due to threshold voltage as described above.Therefore, the charge pump according to this embodiment mode isextremely effective in the case of using thin film transistors having athreshold voltage higher than transistors formed on a silicon wafer.

Either or both of the first capacitor element 103 and the secondcapacitor element 123 may be formed integrally with the display device.The integral formation with the display device allows reduction in thenumber of components. On the other hand, if the capacitor element is notformed integrally with the display device, the capacitance of thecapacitor element can be increased. The first capacitor element 103 isrequired to have higher capacitance than the second capacitor element123. Thus, the smaller second capacitor element 123 may be formedintegrally with the display device, thereby the number of components isreduced and cost reduction is achieved. Meanwhile, the larger firstcapacitor element 103 may be formed separately from the display device,thereby the capacitance of the first capacitor element 103 can beincreased.

The charge pump including thin film transistors can be formed integrallywith a liquid crystal display device, a light emitting device and otherdisplay devices.

Although the first transistor 121 has N-type conductivity and the secondtransistor 122 has P-type conductivity in this embodiment mode, theconductivity of the transistors is not exclusively limited. For example,a circuit configuration where the first transistor 121 has P-typeconductivity and the second transistor 122 has N-type conductivity, andthe input of the diode is maintained at a potential of 0 V may beadopted as well. In this case, the direction of the diode 116 isdesirably reversed to that shown in FIG. 4A. That is, in this embodimentmode, the conductivity of the transistor can be changed depending onwhether the input of the diode is maintained at a high level potentialor a low level potential.

Embodiment Mode 5

Described in this embodiment mode are configuration and operation of thecharge pump, which are different from those shown in Embodiment Modes 1to 4. In this embodiment mode, a circuit configuration that can be usedfor the second or later stage is described as is in Embodiment Mode 4.

A charge pump shown in FIG. 5 includes the first transistor 121, thesecond transistor 122, the first capacitor element 103, the secondcapacitor element 123, the inverter 105, and the diode 116. In FIG. 5,the capacitor element 103 corresponds to the capacitor element C1 inFIG. 13 and the diode 116 has a function corresponding to the diode D1in FIG. 13. The first transistor 121, the second transistor 122 and thesecond capacitor element 123 collectively function as the diode D2 inFIG. 13.

It is assumed that a high level potential is Vdd while a low levelpotential is 0 V for simplicity, though the invention is not limited tothis. Accordingly, Vdd is inputted to and outputted from the inverter105 as a high signal while 0 V is inputted to and outputted from theinverter 105 as a low signal. In this embodiment mode, the firsttransistor 121 has N-type conductivity and the second transistor 122 hasP-type conductivity. The diode 116 may be any one of a PN diode, a PINdiode, a Schottky diode, and a diode connected transistor. If the diodeconnected transistor is used, it may have either N-type conductivity orP-type conductivity. The diode 116 may also have any elementconfiguration and circuit configuration. For example, the circuitconfigurations described in Embodiment Modes 1 to 3 may be adopted forthe diode 116.

The connection between each element is described below.

The input of the diode 116 is inputted to a power supply to bemaintained at a high level potential of Vdd. The output of the inverter105 (point Q) is connected to the gate electrode of the secondtransistor 122 and the other electrode of the first transistor 121(point R) through the second capacitor element 123. The input of theinverter 105 is connected to the output of the diode 116 and oneelectrode of the first transistor 121 (point P) through the gateelectrode of the first transistor 121 and the first capacitor element103. That is, the diode 116 is connected to the first capacitor element103 and one electrode of the first transistor 121 so as to be forwardlybiased.

The operation of the charge pump having such a circuit configuration isdescribed.

A clock signal with a high signal of Vdd and a low signal of 0 V isinputted to the input of the inverter 105 (point Q). For example, when alow signal is inputted to the input of the inverter 105 (point Q), ahigh signal is inputted to the second capacitor element 123 while a lowsignal is inputted to the first capacitor element 103 and the gateelectrode of the first transistor 121. Then, the diode 116 is turned onand Vdd is outputted, thereby the potential at the point P becomes Vddand a predetermined charge is accumulated in the first capacitor element103. At this time, the potential at the point P is Vdd and the gateelectrode of the first transistor 121 is at 0 V, thus the firsttransistor 121 is turned off. Accordingly, the voltage at both ends ofthe second capacitor element 123 is held. Since the gate electrode ofthe second transistor 122 has a high level potential at this time, thesecond transistor 122 is turned off.

When the next clock waveform, namely a high signal is inputted to thepoint Q, a low signal is inputted to the second capacitor element 123and a high signal is inputted to the first capacitor element 103 and thegate electrode of the first transistor 121. Then, a high signal isinputted to the first capacitor element 103, and thus the potential atthe point P rises by Vdd that corresponds to a high signal. Accordingly,the second transistor 122 is turned on since the potential at the sourceelectrode thereof (i.e., the potential at the point P) is 2×Vdd and thegate electrode thereof is at 0 V. As a result, a predetermined currentcorresponding to the potential at the point P can be outputted to Voutto boost the voltage at Vout. At this time, the first transistor 121 ofwhich the gate electrode is at Vdd and the potential at one electrode(i.e., the potential at the point R) is lower is turned on. Accordingly,current flows from the point P to the point R. When the voltage betweenthe point R and the point Q, namely the gate-source voltage of the firsttransistor 121 becomes equal to the threshold voltage (Vth) of the firsttransistor 121, the first transistor 121 is turned off, thereby thepotential at the point R becomes equal to Vdd−Vth. At this time, thevoltage at both ends of the second capacitor element 123 is Vdd−Vth asthe inverter 105 outputs a voltage of 0 V to the second capacitorelement 123.

When a low signal is then inputted to the input of the inverter 105,namely the point Q, a high signal is inputted to the second capacitorelement 123 while a low signal is inputted to the first capacitorelement 103 and the gate electrode of the first transistor 121. Asdescribed above, the potential at the gate electrode of the firsttransistor 121 is 0 V whereas the potential at one electrode thereof(the potential at the point P) is Vdd, thus the first transistor 121 isturned off. Accordingly, the charge in the second capacitor element 123is held. Further, one electrode of the second capacitor element 123 hasa high level potential of Vdd, thus the potential at the point R becomesequal to (Vdd−Vth)+Vdd=2×Vdd−Vth. As a result, the potential at the gateelectrode of the second transistor 122 is 2×Vdd−Vth and the potential atthe point P is Vdd, thus the second transistor 122 is turned off.Accordingly, charge leak from Vout to the point P through the secondtransistor 122 can be prevented. As the diode 116 is turned on, apredetermined charge corresponding to Vdd is accumulated in the firstcapacitor element 103 as described above.

In order to turn the second transistor 122 off certainly, the absolutevalue of the threshold voltage Vth of the first transistor 121 should beset smaller than that of the second transistor 122. This is because thesecond transistor 122 is turned off easily in the case of the potentialat the gate electrode thereof (point R) being high, which becomes equalto 2×Vdd−Vth when a low signal is inputted to the point Q. Here, Vth isthe threshold voltage of the first transistor 121. On the other hand,when the absolute value of the threshold voltage (Vth) of the secondtransistor 122 is smaller than that of the first transistor 121, thepotential at Vout drops by the difference therebetween.

When the second transistor 122 is turned off certainly, the chargeaccumulated in the first capacitor element 103 is preferably not lost.

The potential at Vout can be increased to 2×Vdd by repeating suchoperation.

In the charge pump according to this embodiment mode, when a high signalis inputted to the point Q, the potential at the gate electrode of thesecond transistor 122 (point R) can be lowered using the firsttransistor 121, thereby the potential at the point P can be made equalto Vout. That is, the voltage at Vout does not become 2×Vdd−Vth.Accordingly, a predetermined voltage can be outputted to Voutindependently of the threshold voltage (Vth) of the second transistor122. In other words, in the charge pump according to this embodimentmode, a predetermined voltage can be outputted to Vout without beingaffected by voltage drop due to the threshold voltage of the secondtransistor 122.

Meanwhile, when a low signal is inputted to the point Q, the potentialat the point R (the gate electrode of the second transistor 122) can beincreased using the second capacitor element 123. Therefore, it can beprevented that the potential at Vout is lowered through the secondtransistor 122.

This embodiment mode is not limited to the connection shown in FIG. 5.Different signals may be supplied to the point Q and the capacitorelement 123 instead of providing the inverter 105. In this case,inverted signals are desirably supplied to the point Q and the capacitorelement 123, though the invention is not limited to this. The signalssupplied to the point Q and the capacitor element 123 are notnecessarily inverted as long as the circuit operates normally.

A high signal inputted to the point Q is not necessarily equal to Vdd,and may have a voltage lower or higher than Vdd. Similarly, a low signalinputted to the point Q is not necessarily equal to 0 V, and may have avoltage lower or higher than 0 V.

A high signal inputted to the capacitor element 123 is not necessarilyequal to Vdd, and may have a voltage lower or higher than Vdd.Similarly, a low signal inputted to the capacitor element 123 is notnecessarily equal to 0 V, and may have a voltage lower or higher than 0V.

In the aforementioned charge pump, thin film transistors can be used asthe transistors. As a result, the charge pump can be formed integrallywith a display device or a nonvolatile memory such as a flash memory.When thin film transistors are used in the charge pump, however, it isdifficult to raise the potential to a predetermined level because of ahigh threshold voltage. In addition, variations in threshold voltages ofthin film transistors may cause variations in potentials to beoutputted. Meanwhile, when the charge pump according to this embodimentmode is used, voltage drop due to threshold voltage can be prevented asdescribed above. Therefore, the charge pump according to this embodimentmode is extremely effective in the case of using thin film transistorshaving a threshold voltage higher than transistors formed on a siliconwafer.

The charge pump including thin film transistors can be formed integrallywith a pixel portion of a liquid crystal display device, a lightemitting device and other display devices. At this time, either or bothof the first capacitor element 103 and the second capacitor element 123may be formed integrally with the display device. The integral formationwith the display device allows reduction in the number of components. Onthe other hand, if the capacitor element is not formed integrally withthe display device, the capacitance of the capacitor element can beincreased. The first capacitor element 103 is required to have highercapacitance than the second capacitor element 123. Thus, the smallersecond capacitor element 123 may be formed integrally with the displaydevice, thereby the number of components is reduced and cost reductionis achieved. Meanwhile, the larger first capacitor element 103 may beformed separately from the display device, thereby the capacitance ofthe first capacitor element 103 can be increased.

Although the first transistor has N-type conductivity and the secondtransistor has P-type conductivity in this embodiment mode, theconductivity of the transistors is not exclusively limited. For example,a circuit configuration where the first transistor has P-typeconductivity and the second transistor has N-type conductivity, and theinput of the diode is maintained at a potential of 0 V may be adopted aswell. In this case, the direction of the diode 116 is desirably reversedto that shown in FIG. 5. That is, in this embodiment mode, theconductivity of the transistor can be changed depending on whether theinput of the diode is maintained at a high level potential or a lowlevel potential.

Embodiment Mode 6

Described in this embodiment mode are configuration and operation of thecharge pump, which are different from those shown in Embodiment Modes 1to 5. In this embodiment mode, a circuit configuration that can be usedfor the second or later stage is described as is in Embodiment Modes 4and 5.

A charge pump shown in FIG. 6 includes the first transistor 121, thesecond transistor 122, the first capacitor element 103, the secondcapacitor element 123, the inverter 105, and the diode 116. The firstcapacitor element 103 in FIG. 6 corresponds to the capacitor element C1in FIG. 13 and the diode 116 has a function corresponding to the diodeD1 in FIG. 13. The first transistor 121, the second transistor 122 andthe second capacitor element 123 collectively function as the diode D2in FIG. 13.

It is assumed that a high level potential is Vdd while a low levelpotential is 0 V for simplicity, though the invention is not limited tothis. Accordingly, Vdd is outputted from the inverter 105 as a highsignal while 0 V is outputted from the inverter 105 as a low signal. Inthis embodiment mode, the first transistor 121 has N-type conductivityand the second transistor 122 has P-type conductivity. The diode 116 maybe any one of a PN diode, a PIN diode, a Schottky diode, and a diodeconnected transistor. If the diode connected transistor is used, it mayhave either N-type conductivity or P-type conductivity. The diode 116may also have any element configuration and circuit configuration. Forexample, the circuit configurations described in Embodiment Modes 1 to 3may be adopted for the diode 116.

The connection between each element is described below.

The input of the diode 116 is connected to a power supply to bemaintained at a high level potential of Vdd. The output of the inverter105 is connected to the gate electrode of the second transistor 122 andone electrode of the first transistor 121 (point R) through the secondcapacitor element 123. The input of the inverter 105 (point Q) isconnected to the gate electrode of the first transistor 121 andconnected to one electrode of the second transistor 122 and the outputof the diode 116 (point P) through the first capacitor element 103. Thatis, the diode 116 is connected to the first capacitor element 103 andone electrode of the second transistor 122. The circuit configuration ofthe charge pump shown in this embodiment mode is different from thatshown in Embodiment Mode 5 in that the other electrode of the firsttransistor 121 is connected to the other electrode of the secondtransistor 122.

The operation of the charge pump having such a circuit configuration isdescribed.

A clock signal with a high signal of Vdd and a low signal of 0 V isinputted to the input of the inverter 105 (point Q). For example, when alow signal is inputted to the input of the inverter 105 (point Q), ahigh signal is inputted to the second capacitor element 123 while a lowsignal is inputted to the first capacitor element 103 and the gateelectrode of the first transistor 121. Then, the diode 116 is turned onand Vdd is outputted to the point P while 0 V is inputted to the pointQ, thereby a predetermined charge corresponding to Vdd is accumulated inthe first capacitor element 103. Since the gate electrode of the secondtransistor 122 (point R) has a high level potential at this time, thesecond transistor 122 is turned off. At this time, the first transistor121 of which the gate electrode is at 0 V and one electrode (point R) isat Vdd is turned off.

When the next clock waveform, namely a high signal is inputted to thepoint Q, a low signal is inputted to the second capacitor element 123while a high signal is inputted to the first capacitor element 103 andthe gate electrode of the first transistor 121. The first transistor 121of which the gate electrode (point Q) is at Vdd and the other electrode(point R) has a low level potential is turned on. Accordingly, currentflows from Vout to the point R. Then, when the voltage between the pointQ and the point R, namely the gate-source voltage of the firsttransistor 121 becomes equal to Vth, the first transistor 121 is turnedoff. Since the voltage at the point Q is Vdd at this time, the voltageat the point R becomes Vdd−Vth. Thus, a charge corresponding to Vgs ofthe first transistor 121, namely Vdd−Vth of the first transistor 121 isaccumulated in the second capacitor element 123. A high signal isinputted to the first capacitor element 103, therefore, the voltage atthe point P increases by Vdd corresponding to a high signal. At thistime, the second transistor 122 of which one electrode (point P) is at 0V×Vdd and the gate electrode is at Vdd−Vth is turned on. As a result, apredetermined current corresponding to 2×Vdd can be outputted to Vout,thereby the voltage at Vout is boosted.

When a low signal is then inputted to the input of the inverter 105(point Q), a high signal is inputted to the second capacitor element 123while a low signal is inputted to the first capacitor element 103 andthe gate electrode of the first transistor 121. As described above, thevoltage at the gate electrode of the first transistor 121 is 0 V whereasthe potential at one electrode thereof (the potential at the point P) isVdd, thus the first transistor 121 is turned off. Accordingly, thecharge in the second capacitor element 123 is held. Further, oneelectrode of the second capacitor element 123 has a high level potentialof Vdd, thus the potential at the point R becomes equal to(Vdd−Vth)+Vdd=2×Vdd−Vth. As a result, the voltage at the gate electrodeof the second transistor 122 is 2×Vdd−Vth and Vout is at 0 V×Vdd, thusthe second transistor 122 is turned off. As the diode 116 is turned onand the potential at the point P is Vdd, a predetermined chargecorresponding to Vdd is accumulated in the first capacitor element 103as described above.

In order to turn the second transistor 122 off certainly, the absolutevalue of the threshold voltage Vth of the first transistor 121 should beset smaller than that of the second transistor 122. This is because thesecond transistor 122 is turned off easily in the case of the potentialat the gate electrode thereof (point R) being high, which becomes equalto 2×Vdd−Vth when a low signal is inputted to the point Q. Here, Vth isthe threshold voltage of the first transistor 121. On the other hand,when the absolute value of the threshold voltage (Vth) of the secondtransistor 122 is smaller than that of the first transistor 121, thepotential at Vout drops by the difference therebetween.

When the second transistor 122 is turned off certainly, the accumulatedcharge of 2×Vdd is preferably not lost.

The potential at Vout can be increased to 2×Vdd by repeating suchoperation.

In the charge pump according to this embodiment mode, a predeterminedcharge can be outputted to Vout independently of the threshold voltage(Vth) of the second transistor 122. In other words, in the charge pumpaccording to this embodiment mode, a predetermined charge can beaccumulated without being affected by voltage drop due to the thresholdvoltage of the second transistor 122.

This embodiment mode is not limited to the connection shown in FIG. 6.Different signals may be supplied to the point Q and the capacitorelement 123 instead of providing the inverter 105. In this case,inverted signals are desirably supplied to the point Q and the capacitorelement 123, though the invention is not limited to this. The signalssupplied to the point Q and the capacitor element 123 are notnecessarily inverted as long as the circuit operates normally.

A high signal inputted to the point Q is not necessarily equal to Vdd,and may have a voltage lower or higher than Vdd. Similarly, a low signalinputted to the point Q is not necessarily equal to 0 V, and may have avoltage lower or higher than 0 V.

A high signal inputted to the capacitor element 123 is not necessarilyequal to Vdd, and may have a voltage lower or higher than Vdd.Similarly, a low signal inputted to the capacitor element 123 is notnecessarily equal to 0 V, and may have a voltage lower or higher than 0V.

In the aforementioned charge pump, thin film transistors can be used asthe transistors. As a result, the charge pump can be formed integrallywith a display device or a nonvolatile memory such as a flash memory.When thin film transistors are used in the charge pump, however, it isdifficult to raise the potential to a predetermined level because of ahigh threshold voltage. In addition, variations in threshold voltages ofthin film transistors may cause variations in potentials to beoutputted. Meanwhile, when the charge pump according to this embodimentmode is used, voltage drop due to threshold voltage can be prevented asdescribed above. Therefore, the charge pump according to this embodimentmode is extremely effective in the case of using thin film transistorshaving a threshold voltage higher than transistors formed on a siliconwafer.

The charge pump including thin film transistors can be formed integrallywith a pixel portion of a liquid crystal display device, a lightemitting device and other display devices. At this time, either or bothof the first capacitor element 103 and the second capacitor element 123may be formed integrally with the display device. The integral formationwith the display device allows reduction in the number of components. Onthe other hand, if the capacitor element is not formed integrally withthe display device, the capacitance of the capacitor element can beincreased. The first capacitor element 103 is required to have highercapacitance than the second capacitor element 123. Thus, the smallersecond capacitor element 123 may be formed integrally with the displaydevice, thereby the number of components is reduced and cost reductionis achieved. Meanwhile, the larger first capacitor element 103 may beformed separately from the display device, thereby the capacitance ofthe first capacitor element 103 can be increased.

Although the first transistor has N-type conductivity and the secondtransistor has P-type conductivity in this embodiment mode, theconductivity of the transistors is not exclusively limited. For example,a circuit configuration where the first transistor has P-typeconductivity and the second transistor has N-type conductivity, and theinput of the diode is maintained at a potential of 0 V may be adopted aswell. In this case, the direction of the diode 116 is desirably reversedto that shown in FIG. 6. That is, in this embodiment mode, theconductivity of the transistor can be changed depending on whether theinput of the diode is maintained at a high level potential or a lowlevel potential.

Embodiment Mode 7

Described in this embodiment mode are configuration and operation of thecharge pump, which are different from those shown in Embodiment Modes 1to 6. In this embodiment mode, a circuit configuration that can be usedfor the second or later stage is described as is in Embodiment Modes 4to 6.

A charge pump shown in FIG. 7A includes a first transistor 131, a secondtransistor 132, a third transistor 133, the capacitor element 103, andthe diode 116. That is, the charge pump in this embodiment mode does notinclude an inverter. The capacitor element 103 in FIG. 7A corresponds tothe capacitor element C1 in FIG. 13 and the diode 116 has a functioncorresponding to the diode D1 in FIG. 13. The first transistor 131, thesecond transistor 132 and the third transistor 133 collectively functionas the diode D2 in FIG. 13.

It is assumed that a high level potential is Vdd while a low levelpotential is 0 V for simplicity, though the invention is not limited tothis. Accordingly, Vdd is inputted to the point Q as a high signal while0 V is inputted to the point Q as a low signal. In this embodiment mode,the first transistor 131 and the second transistor 132 have P-typeconductivity and the third transistor 133 has N-type conductivity. Thediode 116 may be any one of a PN diode, a PIN diode, a Schottky diode,and a diode connected transistor. If the diode connected transistor isused, it may have either N-type conductivity or P-type conductivity. Thediode 116 may also have any element configuration and circuitconfiguration. For example, the circuit configurations described inEmbodiment Modes 1 to 3 may be adopted for the diode 116.

The connection between each element is described below.

The input of the diode 116 is connected to a power supply to bemaintained at a high level potential of Vdd. The output of the diode 116(point P) is connected to the gate electrode of the first transistor 131and one electrode of the second transistor 132, and connected to thegate electrode of the third transistor 133 (point Q) through thecapacitor element 103. That is, the diode 116 is connected to thecapacitor element 103, the gate electrode of the first transistor 131and one electrode of the second transistor 132. One electrode of thefirst transistor 131 is connected to the other electrode of the secondtransistor 132, whereas the other electrode of the first transistor 131is connected to one electrode of the third transistor 133 (point R). Theother electrode of the third transistor 133 has a voltage of 0 V.

The operation of the charge pump having such a configuration isdescribed.

A clock signal with a high signal of Vdd and a low signal of 0 V isinputted to the point Q. For example, when a high signal is inputted tothe point Q, a high signal is inputted to the capacitor element 103 andthe gate electrode of the third transistor 133. At this time, the thirdtransistor 133 of which the gate electrode is at Vdd and one electrodeis at 0 V is turned on. The second transistor 132 of which the gateelectrode (point R) is at 0 V is turned on. Thus, the potential at theother electrode of the second transistor 132, namely Vout becomes equalto the potential at the point P. As a result, a predetermined currentaccumulated in the capacitor element 103 can be outputted to Vout,thereby the voltage at Vout is boosted. Since the potential at Vout isequal to that at the point P, the first transistor 131 is turned off.

When the next clock waveform, namely a low signal is inputted to thepoint Q, a low signal is inputted to the capacitor element 103 and thegate electrode of the third transistor 133. At this time, the thirdtransistor 133 of which the gate electrode point Q) is at 0 V and oneelectrode is at 0 V is turned off. Further, Vdd is outputted from thediode 116 to the point P, thereby a charge corresponding to Vdd isaccumulated in the capacitor element 103. The first transistor 131 ofwhich the gate electrode (point P) is at Vdd and one electrode is at 0V×Vdd is turned on. As a result, the potential at Vout becomes equal tothat at the point R, thus the second transistor 132 is turned off.Accordingly, charge leak from Vout to the point P through the secondtransistor 132 can be prevented.

When the next clock waveform, namely a high signal is then inputted tothe point Q, a high signal is inputted to the capacitor element 103 andthe gate electrode of the third transistor 133. At this time, the thirdtransistor 133 of which the gate electrode is at Vdd and one electrodeis at 0 V is turned on. The second transistor 132 of which the gateelectrode (point R) is at 0 V is turned on. Thus, the potential at theother electrode of the second transistor 132, namely Vout becomes equalto the potential at the point P, namely 2×Vdd. As a result, apredetermined current accumulated in the capacitor element 103 can beoutputted to Vout, thereby the voltage at Vout is boosted. Since thepotential at Vout is equal to that at the point P, the first transistor131 is turned off.

The potential at Vout can be increased to 2×Vdd by repeating suchoperation.

In the charge pump according to this embodiment mode, a predeterminedcharge can be outputted to Vout independently of the threshold voltage(Vth) of the second transistor 132. That is, in the charge pumpaccording to this embodiment mode, a predetermined charge can beaccumulated without being affected by voltage drop due to the thresholdvoltage of the second transistor 132.

Different signals may be supplied to the capacitor element 103 and thegate electrode of the third transistor 133. In this case, the samesignal is desirably supplied to the capacitor element 103 and the gateelectrode of the third transistor 133, though the invention is notlimited to this. Different timing or voltage signals may be supplied tothe capacitor element 103 and the gate electrode of the third transistor133 as long as the circuit operates normally.

A high signal inputted to the capacitor element 103 is not necessarilyequal to Vdd, and may have a voltage lower or higher than Vdd.Similarly, a low signal inputted to the capacitor element 103 is notnecessarily equal to 0 V, and may have a voltage lower or higher than 0V.

A high signal inputted to the gate electrode of the third transistor 133is not necessarily equal to Vdd, and may have a voltage lower or higherthan Vdd. Similarly, a low signal inputted to the gate electrode of thethird transistor 133 is not necessarily equal to 0 V, and may have avoltage lower or higher than 0 V.

A high signal inputted to the gate electrode of the third transistor 133and a high signal inputted to the capacitor element 103 may havedifferent potentials. Similarly, a low signal inputted to the gateelectrode of the third transistor 113 and a low signal inputted to thecapacitor element 103 may have different potentials.

A signal inputted to the source electrode of the third transistor 133 isnot necessarily equal to 0 V, and may have a voltage lower or higherthan 0 V.

Although the circuit shown in FIG. 7A is applied to the second stage inthis embodiment mode, it may be applied to the third or later stage aswell. FIG. 7B shows an example of the circuit applied to the thirdstage. The diode 150 corresponds to the diode D1 in FIG. 13 and thecapacitor element 153 corresponds to the capacitor element C1 in FIG.13.

Different signals may be supplied to the capacitor element 153 and thecapacitor element 103 instead of providing the inverter 105. In thiscase, inverted signals are desirably supplied to the capacitor element153 and the capacitor element 103, though the invention is not limitedto this. The signals supplied to the capacitor element 153 and thecapacitor element 103 are not necessarily inverted as long as thecircuit operates normally.

A high signal inputted to the capacitor element 153 is not necessarilyequal to Vdd, and may have a voltage lower or higher than Vdd.Similarly, a low signal inputted to the capacitor element 153 is notnecessarily equal to 0 V, and may have a voltage lower or higher than 0V.

A high signal inputted to the capacitor element 103 is not necessarilyequal to Vdd, and may have a voltage lower or higher than Vdd.Similarly, a low signal inputted to the capacitor element 103 is notnecessarily equal to 0 V, and may have a voltage lower or higher than 0V.

In the aforementioned charge pump, thin film transistors can be used asthe transistors. As a result, the charge pump can be formed integrallywith a display device or a nonvolatile memory such as a flash memory.When thin film transistors are used in the charge pump, however, it isdifficult to raise the potential to a predetermined level because of ahigh threshold voltage. In addition, variations in threshold voltages ofthin film transistors may cause variations in potentials to beoutputted. Meanwhile, when the charge pump according to this embodimentmode is used, voltage drop due to threshold voltage can be prevented asdescribed above. Therefore, the charge pump according to this embodimentmode is extremely effective in the case of using thin film transistorshaving a threshold voltage higher than transistors formed on a siliconwafer.

The charge pump including thin film transistors can be formed integrallywith a pixel portion of a liquid crystal display device, a lightemitting device and other display devices. At this time, either or bothof the first capacitor element 103 and the second capacitor element 153may be formed integrally with the display device. The integral formationwith the display device allows reduction in the number of components. Onthe other hand, if the capacitor element is not formed integrally withthe display device, the capacitance of the capacitor element can beincreased. The first capacitor element 103 is required to have highercapacitance than the second capacitor element 153. Thus, the smallersecond capacitor element 153 may be formed integrally with the displaydevice, thereby the number of components is reduced and cost reductionis achieved. Meanwhile, the larger first capacitor element 103 may beformed separately from the display device, the capacitance of the firstcapacitor element 103 can be increased.

Although the first transistor 131 and the second transistor 132 haveP-type conductivity and the third transistor 133 has N-type conductivityin this embodiment mode, the conductivity of the transistors is notexclusively limited. For example, a circuit configuration where thefirst transistor 131 and the second transistor 132 have N-typeconductivity and the third transistor 133 has P-type conductivity, andthe input of the diode is maintained at a low level potential of 0 V maybe adopted as well. In this case, the direction of the diode 116 isdesirably reversed to that shown in FIG. 7A. That is, in this embodimentmode, the conductivity of the transistor can be changed depending onwhether the input of the diode is maintained at a high level potentialor a low level potential.

Embodiment Mode 8

Described in this embodiment mode are configuration and operation of thecharge pump where the circuit configuration described in Embodiment Mode1 that can be used for the first stage is combined with the circuitconfiguration described in Embodiment Mode 4 that can be used for thesecond or later stage.

A charge pump shown in FIG. 8 includes the first transistor 101, thesecond transistor 102, the third transistor 121, the fourth transistor122, the first capacitor element 103, the second capacitor element 104,the third capacitor element 123, and the inverter 105. The configurationin FIG. 8 can be obtained by combining FIG. 1A and FIG. 4A. In such acombined charge pump as shown in this embodiment mode, the inverter canbe shared. The first capacitor element 103 in FIG. 8 corresponds to thecapacitor element C1 in FIG. 13, and the first transistor 101, thesecond transistor 102 and the second capacitor element 104 correspond tothe diode D1. The third transistor 121, the fourth transistor 122 andthe third capacitor element 123 correspond to the diode D2 in FIG. 13.It is assumed that a high level potential is Vdd while a low levelpotential is 0 V for simplicity, though the invention is not limited tothis. Accordingly, Vdd is outputted from the inverter 105 as a highsignal while 0 V is outputted from the inverter 105 as a low signal. Inthis embodiment mode, the first to third transistors 101, 102 and 121have N-type conductivity and the fourth transistor 122 has P-typeconductivity.

The connection between each element is described hereinafter.

One electrode of the first transistor 101 (point S) is connected to apower supply to be maintained at a high level potential of Vdd andconnected to one electrode of the third transistor 121. The output ofthe inverter 105 is connected to the gate electrode of the secondtransistor 102 and connected to the gate electrode of the thirdtransistor 121 and one electrode of the fourth transistor 122 (point R)through the first capacitor element 103. The input of the inverter 105(point Q) is connected to the gate electrode of the first transistor 101and one electrode of the second transistor 102 (point P) though thesecond capacitor element 104, and connected to the gate electrode of thefourth transistor 122 and the other electrode of the third transistor121 (point T) through the third capacitor element 123. The otherelectrode of the first transistor 101 is connected to the gate electrodeof the third transistor and one electrode of the fourth transistor 122.The other electrode of the second transistor 102 is connected to theother electrode of the fourth transistor 122.

The operation of the charge pump having such a circuit configuration issimilar to that shown in Embodiment Modes 1 and 7, therefore thedescription thereof is omitted herein.

As set forth above, the circuit configurations described in EmbodimentModes 1 to 3 and the circuit configurations described in EmbodimentModes 4 to 7 can be combined freely.

FIG. 18 shows a layout example to obtain the charge pump shown in FIG.8. The first transistor 101 and the fourth transistor 122 have a largerchannel width than the second transistor 102 and the third transistor121.

The first capacitor element 103, the second capacitor element 104 andthe third capacitor element 123 can be constituted by a semiconductorfilm added with an N-type impurity, an insulating film such as a gateinsulating film, and a conductive film to be a gate electrode, or aconductive film to be a gate electrode, an insulating film such as aninterlayer insulating film, and a conductive film to be a wiring. Themobility of charges in the semiconductor film is lower than in theconductive film. Therefore, in the first capacitor element 103, theconductive film has a comb shape so that charges may move accuratelyeven in the center of the semiconductor film. The semiconductor film andthe wiring are connected to each other through a number of contact holesformed in the interlayer insulating film and the gate insulating film.As a result, the capacitor elements sharing the conductive film to be agate electrode are connected in parallel, leading to increasedcapacitance.

FIG. 19 shows a cross sectional view of FIG. 18 along lines A-A′ andB-B′ and a pixel portion formed integrally therewith.

In the first capacitor element 103, the first transistor 101 and thepixel portion, thin film transistors 601 and 101 are formed, where asemiconductor film 602, a gate insulating film 603 covering thesemiconductor film 602, a gate electrode 605, an impurity region formedutilizing the gate electrode 605 in a self aligned manner, and a wiring606 connected to the impurity region are formed over an insulatingsubstrate 600 with a base film interposed therebetween. The thin filmtransistor is used as the first transistor 101. An interlayer insulatingfilm 605 is formed to improve planarity. The interlayer insulating film605 is formed of an inorganic material or an organic material and has asingle layer structure or a multilayer structure.

A first electrode 607 connected to the wiring 606, an electroluminescentlayer 609, and a second electrode 610 are formed, which collectivelyconstitute a light emitting element 612. At this time, a separationlayer 608 formed of an insulating film is formed so as to discriminatethe electroluminescent layer 609.

The insulating film is formed of an inorganic material or an organicmaterial and has a single layer structure or a multilayer structure. Inthe capacitor element 103 region, the wiring 606 is connected to thesemiconductor film 602 through a contact hole formed in the interlayerinsulating film 605 and the gate insulating film 603. In this manner,the capacitor element 103 capable of holding large capacitance can bemanufactured.

Instead of forming the impurity region utilizing the gate electrode in aself aligned manner, the impurity region may be formed in the entiresemiconductor film constituting the capacitor element 103.

Subsequently, a counter substrate 610 is attached. If a space 613 isgenerated by attaching the counter substrate 610, it is preferablyfilled with gas such as nitrogen in order to prevent moisture thatcauses degradation of the light emitting element from entering.Alternatively, the space 613 may be filled with an adhesive such asresin. A light emitting device is thus completed.

In the charge pump according to this embodiment mode, a predeterminedcharge can be outputted to Vout independently of the threshold voltage(Vth) of the first transistor 101 and the second transistor 102. Thatis, in the charge pump according to this embodiment mode, a voltage canbe outputted to Vout without being affected by voltage drop due to thethreshold voltage of the first transistor 101 and the second transistor102.

In the aforementioned charge pump, thin film transistors can be used asthe transistors. As a result, the charge pump can be formed integrallywith a display device or a nonvolatile memory such as a flash memory.When thin film transistors are used in the charge pump, however, it isdifficult to raise the potential to a predetermined level because of ahigh threshold voltage. In addition, variations in threshold voltages ofthin film transistors may cause variations in potentials to beoutputted. Meanwhile, when the charge pump according to this embodimentmode is used, voltage drop due to threshold voltage can be prevented asdescribed above. Therefore, the charge pump according to this embodimentmode is extremely effective in the case of using thin film transistorshaving a threshold voltage higher than transistors formed on a siliconwafer.

The charge pump including thin film transistors can be formed integrallywith a pixel portion of a liquid crystal display device, a lightemitting device and other display devices. At this time, either or bothof the first capacitor element 103 and the second capacitor element 104may be formed integrally with the display device. The integral formationwith the display device allows reduction in the number of components. Onthe other hand, if the capacitor element is not formed integrally withthe display device, the capacitance of the capacitor element can beincreased. The first capacitor element 103 is required to have highercapacitance than the second capacitor element 104. Thus, the smallersecond capacitor element 104 may be formed integrally with the displaydevice, thereby the number of components is reduced and cost reductionis achieved. Meanwhile, the larger first capacitor element 103 may beformed separately from the display device, thereby the capacitance ofthe first capacitor element 103 can be increased.

Although the first to third transistors have N-type conductivity and thefourth transistor has P-type conductivity in this embodiment mode, theconductivity of the transistors is not exclusively limited. For example,a circuit configuration where the first to third transistors have P-typeconductivity and the fourth transistor has N-type conductivity, and oneelectrode of the first transistor is maintained at a low level potentialof 0 V may be adopted as well. That is, in this embodiment mode, theconductivity of the transistor can be changed depending on whether oneelectrode of the first transistor is maintained at a high levelpotential or a low level potential.

A charge pump can be configured by combining the booster circuitsdescribed in the aforementioned embodiment modes. For example, thefollowing charge pumps can be obtained.

FIG. 20 shows a circuit configuration of a charge pump configured by thecircuit shown in FIG. 1A that can be used for the first stage and thecircuit shown in FIG. 5 that can be used for the second or later stage.

FIG. 21 shows a circuit configuration of a charge pump configured by thecircuit shown in FIG. 1A that can be used for the first stage and thecircuit shown in FIG. 6 that can be used for the second or later stage.

FIG. 22 shows a circuit configuration of a charge pump configured by thecircuit shown in FIG. 1A that can be used for the first stage and thecircuit shown in FIG. 7 that can be used for the second or later stage.

FIG. 23 shows a circuit configuration of a charge pump configured by thecircuit shown in FIG. 2 that can be used for the first stage and thecircuit shown in FIG. 4 that can be used for the second or later stage.

FIG. 24 shows a circuit configuration of a charge pump configured by thecircuit shown in FIG. 2 that can be used for the first stage and thecircuit shown in FIG. 5 that can be used for the second or later stage.

FIG. 25 shows a circuit configuration of a charge pump configured by thecircuit shown in FIG. 2 that can be used for the first stage and thecircuit shown in FIG. 6 that can be used for the second or later stage.

FIG. 26 shows a circuit configuration of a charge pump configured by thecircuit shown in FIG. 2 that can be used for the first stage and thecircuit shown in FIG. 7 that can be used for the second or later stage.

FIG. 27 shows a circuit configuration of a charge pump configured by thecircuit shown in FIG. 3 that can be used for the first stage and thecircuit shown in FIG. 4 that can be used for the second or later stage.

FIG. 28 shows a circuit configuration of a charge pump configured by thecircuit shown in FIG. 3 that can be used for the first stage and thecircuit shown in FIG. 5 that can be used for the second or later stage.

FIG. 29 shows a circuit configuration of a charge pump configured by thecircuit shown in FIG. 3 that can be used for the first stage and thecircuit shown in FIG. 6 that can be used for the second or later stage.

FIG. 30 shows a circuit configuration of a charge pump configured by thecircuit shown in FIG. 3 that can be used for the first stage and thecircuit shown in FIG. 7 that can be used for the second or later stage.

In this manner, the circuit used for the first stage and the circuitused for the second or later stage can be combined freely.

Embodiment Mode 9

Described in this embodiment mode are configuration and operation of thecharge pump, which are different from those shown in Embodiment Modes 1to 8. In this embodiment mode, a circuit configuration that can be usedfor the first stage as is in Embodiment Mode 1 is described, where oneelectrode of the first transistor is maintained at a low level potentialof 0 V.

FIG. 14 shows a circuit configuration of a four-stage Dickson chargepump. The direction of the diode in FIG. 14 is reversed to that of theDickson charge pump shown in FIG. 13. Accordingly, a high levelpotential at the negative side can be generated.

FIG. 9 shows a charge pump where the conductivity of the transistors andthe direction of the diode 106 are opposite to those in FIG. 1A. Thecharge pump shown in FIG. 9 includes, as in FIG. 1A, the firsttransistor 101, the second transistor 102, the first capacitor element103, the second capacitor element 104, the inverter 105, and the diode106. The first transistor 101, the second transistor 102 and the secondcapacitor element 104 collectively function as the diode D1 in FIG. 13.The first capacitor element 103 in FIG. 9 corresponds to the capacitorelement C1 in FIG. 14, and the diode 106 corresponds to the diode D2 inFIG. 14. It is assumed that a low level potential is 0 V for simplicity,though the invention is not limited to this. A high level potential isassumed to be Vdd. Accordingly, Vdd is inputted to and outputted fromthe inverter 105 as a high signal while 0 V is inputted to and outputtedfrom the inverter 105 as a low signal. In this embodiment mode, thefirst transistor 101 and the second transistor 102 have P-typeconductivity. The diode 106 may be any one of a PN diode, a PIN diode, aSchottky diode, and a diode connected transistor. If the diode connectedtransistor is used, it may have either N-type conductivity or P-typeconductivity. The diode 106 may also have any circuit configuration.

The connection between each element and the operation thereof aresimilar to those in FIG. 1A except that one electrode of the firsttransistor 101 is connected to a power supply to be maintained at a lowlevel potential of 0 V and the direction of the diode is reversed. Sucha connection allows −VDD to be outputted to Vout.

The operation of the charge pump having such a configuration isdescribed.

A clock signal with a high signal of Vdd and a low signal of 0 V isinputted to the input of the inverter 105 (point Q). If a low signal isinputted to the input of the inverter 105 (point Q), for example, a highsignal is inputted to the second capacitor element 104 while a lowsignal is inputted to the gate electrode of the second transistor 102and the first capacitor element 103. Since a high signal is inputted tothe second capacitor element 104, the potential at the point R rises.Then, the second transistor 102 of which the gate electrode is at 0 V isturned on, and thus current flows from the point R to Vout. When thevoltage between the point Q and the point R, namely the gate-sourcevoltage of the second transistor 102 becomes equal to Vth of the secondtransistor 102, the second transistor 102 is turned off. Therefore, thepotential at the point R is lower than that at the point Q by Vth(higher by |Vth|). Note that Vth is indicated by a negative value sincethe second transistor 102 has P-type conductivity. Thus, the potentialat the point R is |Vth|(−Vth). The first transistor of which oneelectrode is at 0 V and the gate electrode is |Vth| is turned off. Atthis time, the diode 106 is turned on and the potential at the point Pbecomes equal to Vout.

When the next clock waveform, namely a high signal is inputted to thepoint Q, a high signal is inputted to the gate electrode of the secondtransistor 102 and the first capacitor element 103, while a low signalis inputted to the second capacitor element 104. Then, a charge of −Vddcorresponding to a low signal is accumulated in the second capacitorelement 104 in addition to a predetermined charge that has beenpreviously accumulated. Meanwhile, a charge of Vdd corresponding to ahigh signal is accumulated in the first capacitor element 103 inaddition to a predetermined charge that has been previously accumulated.At this time, the second transistor 102 of which the gate electrode isat Vdd is turned off, thereby the charge in the second capacitor element104 is held. Since 0 V is inputted to the point S, the potential at thepoint R drops by Vdd and becomes equal to |Vth|−Vdd. Thus, the potentialat the gate electrode of the first transistor 101 (point R) becomesequal to |Vth|−Vdd, thereby the first transistor 101 is turned on. As aresult, the potential at the point P becomes 0 V and Vdd is inputted tothe point Q, thus a charge of −Vdd is accumulated in the first capacitorelement 103.

When the next clock waveform, namely a low signal is then inputted tothe point Q, a high signal is inputted to the second capacitor element104 while a low signal is inputted to the gate electrode of the secondtransistor 102 and the first capacitor element 103. The secondtransistor of which one electrode (Vout) is at −Vdd and the gateelectrode is at 0 V is turned on. A predetermined charge is accumulatedin the second capacitor element 104 until it becomes equal to thethreshold voltage Vth of the second transistor 102. Thus, the potentialat the point R becomes |Vth|. The first transistor 101 of which oneelectrode is at 0 V and the gate electrode is at |Vth| is turned off.The potential of the first capacitor element 103 (point P) drops by −Vddcorresponding to a low signal. At this time, the potential at the outputof the diode 106 (point P) is lower that that at the input thereof(Vout), therefore, a predetermined current, namely a current of −Vdd isoutputted to Vout and Vout is boosted.

By repeating such operation, the potential at Vout can be made −Vdd.

In the charge pump according to this embodiment mode also, voltage drop(voltage rise) due to the threshold voltage of the first transistor 101can be prevented by the second capacitor element 104 and the secondtransistor 102.

In the aforementioned charge pump, thin film transistors can be used asthe transistors. As a result, the charge pump can be formed integrallywith a display device or a nonvolatile memory such as a flash memory.When thin film transistors are used in the charge pump, however, it isdifficult to raise the potential to a predetermined level because of ahigh threshold voltage. In addition, variations in threshold voltages ofthin film transistors may cause variations in potentials to beoutputted. Meanwhile, when the charge pump according to this embodimentmode is used, voltage drop due to threshold voltage can be prevented bythe second capacitor element 104 as described above. Therefore, thecharge pump according to this embodiment mode is extremely effective inthe case of using thin film transistors having a threshold voltagehigher than transistors formed on a silicon wafer.

The charge pump including thin film transistors can be formed integrallywith a pixel portion of a liquid crystal display device, a lightemitting device and other display devices. At this time, either or bothof the first capacitor element 103 and the second capacitor element 104may be formed integrally with the display device. The integral formationwith the display device allows reduction in the number of components. Onthe other hand, if the capacitor element is not formed integrally withthe display device, the capacitance of the capacitor element can beincreased. The first capacitor element 103 is required to have highercapacitance than the second capacitor element 104. Thus, the smallersecond capacitor element 104 may be formed integrally with the displaydevice, thereby the number of components is reduced and cost reductionis achieved. Meanwhile, the larger first capacitor element 103 may beformed separately from the display device, thereby the capacitance ofthe first capacitor element 103 can be increased.

In this manner, a circuit for dropping the voltage at Vout can beconfigured easily by making the conductivity of the transistors and thedirection of the diode 106 opposite to the circuit for boosting Vout.Therefore, the circuits shown in FIGS. 2 and 3 can also be applied to acircuit for dropping the voltage.

FIG. 31 shows a circuit configuration for dropping the voltage, whichcorresponds to FIG. 2. FIG. 32 shows a circuit configuration fordropping the voltage, which corresponds to FIG. 3.

These circuits for dropping the voltage can be obtained only by changingthe conductivity of transistor and the direction of diode, thus, thecircuit configurations described in Embodiment Modes 1 to 3 can beapplied to the circuit for dropping the voltage.

Embodiment Mode 10

Described in this embodiment mode are configuration and operation of thecharge pump, which are different from those shown in Embodiment Modes 1to 9. In this embodiment mode, a circuit configuration that can be usedfor the second or later stage as is in Embodiment Mode is described,where one electrode of the first transistor is maintained at a low levelpotential of 0 V.

FIG. 10 shows a charge pump where the polarity of the transistors andthe direction of the diode 116 are reversed to those in FIG. 4. Thecharge pump in FIG. 10 includes, as in FIG. 4, the first transistor 121,the second transistor 122, the first capacitor element 103, the secondcapacitor element 123, the inverter 105, and the diode 116. Thecapacitor element 103 in FIG. 10 corresponds to the capacitor element C1in FIG. 14, and the diode 116 has a function corresponding to the diodeD2 in FIG. 14. The first transistor 121, the second transistor 122 andthe second capacitor element 123 collectively function as the diode D1in FIG. 14. It is assumed that a low level potential is 0 V forsimplicity, though the invention is not limited to this. A high levelpotential is assumed to be Vdd. Accordingly, Vdd is inputted to andoutputted from the inverter 105 as a high signal while 0 V is inputtedto and outputted from the inverter 105 as a low signal. In thisembodiment mode, the first transistor 121 has P-type conductivity andthe second transistor 122 has N-type conductivity. The diode 116 may beany one of a PN diode, a PIN diode, a Schottky diode, and a diodeconnected transistor. If the diode connected transistor is used, it mayhave either N-type conductivity or P-type conductivity. The diode 116may also have any element configuration and circuit configuration.

The connection between each element and the operation thereof aresimilar to those in FIG. 4 except that one electrode of the firsttransistor 121 is connected to a power supply to be maintained at a lowlevel potential of 0 V and the connection of the diode 116 is reversed.Therefore, the description is omitted in this embodiment mode.

In such a case, when a clock signal with a high signal of Vdd and a lowsignal of 0 V is inputted to the input of the inverter 105, a voltage of−Vdd is outputted to Vout.

In the charge pump according to this embodiment mode also, apredetermined charge can be outputted without being affected by voltagedrop due to threshold voltage. That is, voltage drop due to thethreshold voltage of the second transistor 122 can be prevented. Theoperation of this circuit is similar to that in FIG. 4, therefore thedescription thereof is omitted herein.

In the aforementioned charge pump, thin film transistors can be used asthe transistors. As a result, the charge pump can be formed integrallywith a display device or a nonvolatile memory such as a flash memory.When thin film transistors are used in the charge pump, however, it isdifficult to raise the potential to a predetermined level because of ahigh threshold voltage. In addition, variations in threshold voltages ofthin film transistors may cause variations in potentials to beoutputted. Meanwhile, when the charge pump according to this embodimentmode is used, a predetermined charge can be outputted without beingaffected by voltage drop due to threshold voltage as described above.Therefore, the charge pump according to this embodiment mode isextremely effective in the case of using thin film transistors having athreshold voltage higher than transistors formed on a silicon wafer.

The charge pump including thin film transistors can be formed integrallywith a pixel portion of a liquid crystal display device, a lightemitting device and other display devices. At this time, either or bothof the first capacitor element 103 and the second capacitor element 123may be formed integrally with the display device. The integral formationwith the display device allows reduction in the number of components. Onthe other hand, if the capacitor element is not formed integrally withthe display device, the capacitance of the capacitor element can beincreased. The first capacitor element 103 is required to have highercapacitance than the second capacitor element 123. Thus, the smallersecond capacitor element 123 may be formed integrally with the displaydevice, thereby the number of components is reduced and cost reductionis achieved. Meanwhile, the larger first capacitor element 103 may beformed separately from the display device, thereby the capacitance ofthe first capacitor element 103 can be increased.

In this manner, a circuit for dropping the voltage of Vout can beconfigured easily by making the conductivity of the transistors and thedirection of the diode 116 opposite to the circuit for boosting Vout.Therefore, the circuits shown in FIGS. 5 to 7 can also be applied to acircuit for dropping the voltage.

FIG. 33 shows a circuit configuration for dropping the voltage, whichcorresponds to FIG. 5. FIG. 34 shows a circuit configuration fordropping the voltage, which corresponds to FIG. 7.

These circuits for dropping the voltage can be obtained only by changingthe conductivity of transistor and the direction of diode, thus, thecircuit configurations described in Embodiment Modes 1 to 3 can beapplied to the circuit for dropping the voltage.

Embodiment Mode 11

Described in this embodiment mode are configuration and operation of thecharge pump where the circuit configuration described in Embodiment Mode9 that can be used for the first stage is combined with the circuitconfiguration described in Embodiment Mode 10 that can be used for thesecond or later stage.

A charge pump shown in FIG. 11 includes, as in FIG. 8, the firsttransistor 101, the second transistor 102, the third transistor 121, thefourth transistor 122, the first capacitor element 103, the secondcapacitor element 104, the third capacitor element 123, and the inverter105. In such a combined charge pump as shown in this embodiment mode,the inverter can be shared. The first capacitor element 103 in FIG. 11corresponds to the capacitor element C1 in FIG. 14, and the firsttransistor 101, the second transistor 102 and the second capacitorelement 104 have a function corresponding to the diode D1 in FIG. 14.The third transistor 121, the fourth transistor 122 and the thirdcapacitor element 123 correspond to the diode D2 in FIG. 13. It isassumed that a low level potential is 0 V for simplicity, though theinvention is not limited to this. A high level potential is assumed tobe Vdd. Accordingly, Vdd is inputted to and outputted from the inverter105 as a high signal while 0 V is inputted to and outputted from theinverter 105 as a low signal. In this embodiment mode, the first tothird transistors 101, 102 and 121 have P-type conductivity and thefourth transistor 122 has N-type conductivity.

The connection between each element and the operation thereof aresimilar to those in FIG. 8 except that one electrode of the firsttransistor 101 is connected to a power supply to be maintained at a lowlevel potential of 0 V. Therefore, the description is omitted in thisembodiment mode.

When a clock signal with a high signal of Vdd and a low signal of 0 V isinputted to the input of the inverter 105, −Vdd is outputted to Vout.That is, a charge corresponding to −Vdd is accumulated in the firstcapacitor element 103 and a current of −Vdd is outputted to Vout,thereby the voltage at Vout drops.

As set forth above, the circuit configurations described in EmbodimentModes 1 to 7 can be applied to a circuit for dropping the voltage bychanging the conductivity of transistors and the like. Such a circuitfor dropping the voltage can be configured by combining a circuitconfiguration that can be used for the first stage and a circuitconfiguration that can be used for the second or later stage. Forexample, the following charge pumps can be configured.

FIG. 35 shows a circuit configuration of a charge pump for dropping thevoltage, which is obtained by changing the conductivity of transistorsand the like of the circuit shown in FIG. 1A that can be used for thefirst stage and changing the conductivity of transistors and the like ofthe circuit shown in FIG. 5 that can be used for the second or laterstage.

FIG. 36 shows a circuit configuration of a charge pump for dropping thevoltage, which is obtained by changing the conductivity of transistorsand the like of the circuit shown in FIG. 1A that can be used for thefirst stage and changing the conductivity of transistors and the like ofthe circuit shown in FIG. 6 that can be used for the second or laterstage.

FIG. 37 shows a circuit configuration of a charge pump for dropping thevoltage, which is obtained by changing the conductivity of transistorsand the like of the circuit shown in FIG. 1A that can be used for thefirst stage and changing the conductivity of transistors and the like ofthe circuit shown in FIG. 7 that can be used for the second or laterstage.

FIG. 38 shows a circuit configuration of a charge pump for dropping thevoltage, which is obtained by changing the conductivity of transistorsand the like of the circuit shown in FIG. 2 that can be used for thefirst stage and changing the conductivity of transistors and the like ofthe circuit shown in FIG. 4 that can be used for the second or laterstage.

FIG. 39 shows a circuit configuration of a charge pump for dropping thevoltage, which is obtained by changing the conductivity of transistorsand the like of the circuit shown in FIG. 2 that can be used for thefirst stage and changing the conductivity of transistors and the like ofthe circuit shown in FIG. 5 that can be used for the second or laterstage.

FIG. 40 shows a circuit configuration of a charge pump for dropping thevoltage, which is obtained by changing the conductivity of transistorsand the like of the circuit shown in FIG. 2 that can be used for thefirst stage and changing the conductivity of transistors and the like ofthe circuit shown in FIG. 6 that can be used for the second or laterstage.

FIG. 41 shows a circuit configuration of a charge pump for dropping thevoltage, which is obtained by changing the conductivity of transistorsand the like of the circuit shown in FIG. 2 that can be used for thefirst stage and changing the conductivity of transistors and the like ofthe circuit shown in FIG. 7 that can be used for the second or laterstage.

FIG. 42 shows a circuit configuration of a charge pump for dropping thevoltage, which is obtained by changing the conductivity of transistorsand the like of the circuit shown in FIG. 3 that can be used for thefirst stage and changing the conductivity of transistors and the like ofthe circuit shown in FIG. 4 that can be used for the second or laterstage.

FIG. 43 shows a circuit configuration of a charge pump for dropping thevoltage, which is obtained by changing the conductivity of transistorsand the like of the circuit shown in FIG. 3 that can be used for thefirst stage and changing the conductivity of transistors and the like ofthe circuit shown in FIG. 5 that can be used for the second or laterstage.

FIG. 44 shows a circuit configuration of a charge pump for dropping thevoltage, which is obtained by changing the conductivity of transistorsand the like of the circuit shown in FIG. 3 that can be used for thefirst stage and changing the conductivity of transistors and the like ofthe circuit shown in FIG. 6 that can be used for the second or laterstage.

FIG. 45 shows a circuit configuration of a charge pump for dropping thevoltage, which is obtained by changing the conductivity of transistorsand the like of the circuit shown in FIG. 3 that can be used for thefirst stage and changing the conductivity of transistors and the like ofthe circuit shown in FIG. 7 that can be used for the second or laterstage.

In this manner, the circuit used for the first stage and the circuitused for the second or later stage can be combined freely.

FIG. 46 shows a circuit configuration of the charge pump shown in FIG.8, which is applied to a circuit configuration for dropping the voltage.

In the charge pump according this embodiment mode, a predeterminedcharge can be outputted to Vout independently of the threshold voltage(Vth) of the first transistor 101 and the fourth transistor 122. Thatis, in the charge pump according to this embodiment mode, apredetermined charge can be accumulated without being affected byvoltage drop due to the threshold voltage of the first transistor 101and the fourth transistor 122.

In the aforementioned charge pump, thin film transistors can be used asthe transistors. As a result, the charge pump can be formed integrallywith a display device or a nonvolatile memory such as a flash memory.When thin film transistors are used in the charge pump, however, it isdifficult to raise the potential to a predetermined level because of ahigh threshold voltage. In addition, variations in threshold voltages ofthin film transistors may cause variations in potentials to beoutputted. Meanwhile, when the charge pump according to this embodimentmode is used, a predetermined charge can be outputted without beingaffected by voltage drop due to threshold voltage as described above.Therefore, the charge pump according to this embodiment mode isextremely effective in the case of using thin film transistors having athreshold voltage higher than transistors formed on a silicon wafer.

The charge pump including thin film transistors can be formed integrallywith a pixel portion of a liquid crystal display device, a lightemitting device and other display devices. At this time, either or bothof the first capacitor element 103 and the second capacitor element 104may be formed integrally with the display device. The integral formationwith the display device allows reduction in the number of components. Onthe other hand, if the capacitor element is not formed integrally withthe display device, the capacitance of the capacitor element can beincreased. The first capacitor element 103 is required to have highercapacitance than the second capacitor element 104. Thus, the smallersecond capacitor element 104 may be formed integrally with the displaydevice, thereby the number of components is reduced and cost reductionis achieved. Meanwhile, the larger first capacitor element 103 may beformed separately from the display device, thereby the capacitance ofthe first capacitor element 103 can be increased.

A charge pump can be configured by combining the aforementioned circuitsfor dropping the voltage.

Embodiment Mode 12

As an example of a semiconductor device including a charge pump, aconfiguration of a display device typified by a liquid crystal displaydevice and a light emitting device having self luminous elements isdescribed in this embodiment mode.

In a panel portion of a display device shown in FIG. 12, a pixel portion201, a level shifter 202, a gate driver 203, a source driver 204, and acharge pump 205 are formed on a substrate 200. If a power supply voltageinputted from a power supply is lower than a voltage required forelements in the pixel portion 201, the power supply voltage is boostedby the charge pump 205 to be supplied to the level shifter 202.

In the case where the charge pump is formed by using thin filmtransistors, a capacitor of the charge pump can be formed by a gateelectrode and an impurity-doped semiconductor film provided with a gateinsulating film interposed therebetween.

The charge pump including thin film transistors can be formed integrallyin the pixel portion of a liquid crystal display device, a lightemitting device and other display devices. As a result, the clockfrequency of a switching element using the charge pump can be selecteddepending on a display mode, resulting in lower power consumption.

When using thing film transistors, a semiconductor may be selected froman amorphous semiconductor, a semi-amorphous semiconductor (alsoreferred to as SAS) having an intermediate state between an amorphoussemiconductor and a crystalline semiconductor, a microcrystallinesemiconductor where crystal grains of 0.5 to 20 nm in size can beobserved in an amorphous semiconductor, and a crystalline semiconductor.In particular, a microcrystalline state having crystal grains of 0.5 to20 nm in size is called microcrystal (μc).

In this embodiment mode, a thin film transistor may adopt either a topgate structure where a gate electrode is formed over a semiconductorfilm or a bottom gate structure where a gate electrode is formed under asemiconductor film.

Embodiment Mode 13

Described in this embodiment mode is a circuit for stabilizing an outputpotential from a charge pump circuit, namely a stabilizing power supplycircuit (regulator).

In the simplest case, a large capacitor element is disposed at theoutput of a charge pump. This large capacitor element suppressespotential changes and stabilizes the potential.

The large capacitor element may be formed integrally with a displaydevice, or formed in another element. The integral formation with thedisplay device allows reduction in the number of components. On theother hand, if the capacitor element is not formed integrally with thedisplay device, the capacitance of the capacitor element can beincreased.

Another stabilizing power supply circuit monitors an output potentialfrom a charge pump and controls the operation of a clock signal suppliedto the charge pump so as to make the voltage constant.

That is, a clock pulse (CLK) and an inverted clock pulse (CLKB) does notalways have to be inputted to the charge pump circuit, and the inputthereof may be stopped, for example, when the potential at the outputterminal reaches a certain level.

FIG. 16 is a schematic view showing a configuration in the case ofstopping the input of a clock pulse (CLK) or an inverted clock pulse(CLKB).

A voltage of Vdd is supplied to an input terminal of a charge pump 1801from a constant voltage source 1800, and a boosted voltage can beobtained at an output terminal thereof. A potential detecting circuit1803 detects the potential at the output terminal and outputs a controlsignal when the potential reaches a certain level, thereby the input ofa clock pulse (CLK) or an inverted clock pulse (CLKB) from a clock pulsegeneration circuit 1802 is stopped.

When a clock pulse is supplied, the output potential of the charge pumprises. Meanwhile, potential rise is stopped when a clock signal is notsupplied. An output potential is controlled by utilizing this operation.

Accordingly, the potential can be stabilized and a predeterminedpotential can be outputted.

A configuration example of another stabilizing power supply circuit(regulator) is shown in FIGS. 17A and 17B.

The configuration in FIG. 17A is described. An input terminal in FIG.17A is connected to an output terminal of a charge pump circuit. Thatis, before outputting a voltage, the output terminal of the charge pumpis connected to one terminal of a zener diode 1503 of which the otherterminal is connected to GND. Thus, current flows in the zener diode1503 when an output potential reaches a certain level, and the potentialat the output terminal can be controlled.

A stabilizing power supply circuit 1504 includes a capacitor element1502 and the zener diode 1503.

As the capacitor element 1502, a capacitor element with largeelectrostatic capacitance is employed. Accordingly, the voltage betweentwo electrodes of the capacitor element 1502 can be maintained constant,and a constant potential can be outputted to the output terminal of thestabilizing power supply circuit 1504.

The number of the zener diode 1503 is not limited to one, and aplurality of zener diodes may be arranged in series to control thepotential. For example, a plurality of zener diodes may be arranged inaccordance with the potential. Alternatively, zener diodes withdifferent breakdown potentials may be connected in series to control thepotential.

A stabilizing power supply circuit 1517 shown in FIG. 17B is describedhereinafter.

The stabilizing power supply circuit 1517 includes a capacitor element1512, an amplifier 1513, a first resistor 1515, and a second resistor1516.

The voltage between two electrodes of the capacitor element 1512 is usedas a power supply of the amplifier 1513. A constant voltage is inputtedto a non-inverting input terminal of the amplifier 1513 from a referencepower supply 1514. An inverting input terminal of the amplifier 1513 isconnected to an output terminal through the second resistor 1516 andconnected to a ground power supply GND through the first resistor 1515.As the amplifier 1513, a high gain amplifier is employed.

An output voltage of the amplifier 1513 is resistance-divided by thesecond resistor 1516 and the first resistor 1515, and then inputted tothe inverting input terminal. This voltage value is compared with avoltage value inputted to the non-inverting input terminal from thereference power supply 1514 by the amplifier 1513.

An output voltage V₀ of the amplifier 1513 is represented by thefollowing formula 1, provided that the voltage of the reference powersupply 1514 is V_(r), the resistance value of the first resistor 1515 isR₁ and the resistance value of the second resistor 1516 is R₂.

$\begin{matrix}{V_{0} = {V_{r}\frac{R_{1} + R_{2}}{R_{1}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Consequently, the output potential of the amplifier 1513 can becontrolled by the ratio R₂/R₁ between the resistance values of the firstresistor 1515 and the second resistor 1516. That is, the potential atthe output terminal of the stabilizing power supply circuit 1517 can beselected arbitrarily from the potential of the reference power supply1514 to a high level potential inputted to the input terminal of thestabilizing power supply circuit 1517.

The potential may be outputted from first to fourth output terminalsthrough a smoothing circuit instead of the regulator.

Alternatively, a means for detecting an output potential beforeobtaining an output at the output terminal may be provided. In thatcase, input of a clock signal (CLK) or an inverted clock signal (CLKB)to a second electrode of a capacitor element may be stopped when thepotential reaches a predetermined level.

Embodiment Mode 14

A display device using the charge pump of the invention can be appliedto electronic apparatuses such as a video camera, a digital camera, agoggle type display (head mounted display), a navigation system, anaudio reproducing device (car audio set, audio component and the like),a notebook computer, a game machine, a portable information terminal(mobile computer, mobile phone, portable game machine, electronic bookand the like), and an image reproducing device provided with a recordingmedium (specifically, a device that reproduces a recording medium suchas DVD (Digital Versatile Disc) and includes a display for displayingthe reproduced image). In particular, a portable information terminalthat includes a screen usually seen from an angle and thus requires awide viewing angle is desirably equipped with the display device.Specific examples of these electronic apparatuses are shown in FIGS. 15Ato 15H.

FIG. 15A shows a display device that includes a housing 2001, asupporting base 2002, a display portion 2003, speaker portions 2004, anda video input terminal 2005. The charge pump of the invention can beapplied to a power supply circuit of the display portion 2003, leadingto lower power consumption. As a result, the battery life increases andthe display device can operate for a long time. A liquid crystal displaydevice or a light emitting device can be used for the display device,and the display device includes all the information display devices suchas used for personal computer, TV broadcast receiving, or advertisementdisplay.

FIG. 15B shows a digital still camera (a digital camera) that includes amain body 2101, a display portion 2102, an image receiving portion 2103,operating keys 2104, an external connecting port 2105, and a shutter2106. The charge pump of the invention can be applied to a power supplycircuit of the display portion 2102, leading to lower power consumption.As a result, the battery life increases and the digital still camera canoperate for a long time.

FIG. 15C shows a notebook computer that includes a main body 2201, ahousing 2202, a display portion 2203, a keyboard 2204, an externalconnecting port 2205, and a pointing mouse 2206. The charge pump of theinvention can be applied to a power supply circuit of the displayportion 2203, leading to lower power consumption. As a result, thebattery life increases and the notebook computer can operate for a longtime.

FIG. 15D shows a mobile computer that includes a main body 2301, adisplay portion 2302, a switch 2303, operating keys 2304, and aninfrared port 2305. The charge pump of the invention can be applied to apower supply circuit of the display portion 2302, leading to lower powerconsumption. As a result, the battery life increases and the mobilecomputer can operate for a long time.

FIG. 15E shows a portable image reproducing device provided with arecording medium (specifically, a DVD reproducing device), that includesa main body 2401, a housing 2402, a display portion A 2403, a displayportion B 2404, a recording medium (such as DVD) reading portion 2405,an operating key 2406, and a speaker portion 2407. The display portion A2403 mainly displays image data while the display portion B 2404 mainlydisplays character data. The charge pump of the invention can be appliedto a power supply circuit of the display portion A 2403 and the displayportion B 2404, leading to lower power consumption. As a result, thebattery life increases and the image reproducing device can operate fora long time. The image reproducing device equipped with a recordingmedium includes a home game machine and the like.

FIG. 15F shows a goggle type display (head mounted display) thatincludes a main body 2501, a display portion 2502, and an arm portion2503. The charge pump of the invention can be applied to a power supplycircuit of the display portion 2502, leading to lower power consumption.As a result, the battery life increases and the head mounted display canoperate for a long time.

FIG. 15G shows a video camera that includes a main body 2601, a displayportion 2602, a housing 2603, an external connecting port 2604, a remotecontrol receiving portion 2605, an image receiving portion 2606, abattery 2607, an audio input portion 2608, and operating keys 2609. Thecharge pump of the invention can be applied to a power supply circuit ofthe display portion 2602, leading to lower power consumption. As aresult, the battery life increases and the video camera can operate fora long time.

FIG. 15H shows a mobile phone that includes a main body 2701, a housing2702, a display portion 2703, an audio input portion 2704, an audiooutput portion 2705, an operating key 2706, an external connecting port2707, and an antenna 2708. The charge pump of the invention can beapplied to a power supply circuit of the display portion 2703, leadingto lower power consumption. As a result, the battery life increases andthe mobile phone can operate for a long time. The mobile phone consumesless power if the display portion 2703 displays white characters on theblack background.

As set forth above, the charge pump of the invention can be applied tovarious electronic apparatuses.

This application is based on Japanese Patent Application serial no.2004-080124 filed in Japan Patent Office on Mar. 19, 2004, the contentsof which are hereby incorporated by reference.

1. A semiconductor device for boosting or reducing a potential voltage,comprising a first transistor; a second transistor; a first capacitorelement; a second capacitor element; a rectifying element; and a powersupply terminal, wherein one electrode of the first transistor iselectrically connected to a first wiring, which is connected to thepower supply terminal maintaining a first potential; wherein a gateelectrode of the first transistor is electrically connected to oneelectrode of the second transistor and a first electrode of the firstcapacitor element, wherein a gate electrode of the second transistor anda first electrode of the second capacitor element are electricallyconnected to a second wiring, which is input a second potential; andwherein a second electrode of the second capacitor is electricallyconnected to the other electrode of the first transistor and a firstelectrode of the rectifying element.
 2. The semiconductor deviceaccording to claim 1, wherein the first electrode of the rectifyingelement is electrically connected to the other electrode of the firsttransistor; and wherein a second electrode of the rectifying element iselectrically connected to the other electrode of the second transistor.3. The semiconductor device according to claim 1, wherein the firstelectrode of the rectifying element is electrically connected to theother electrode of the first transistor and the other electrode of thesecond transistor.
 4. The semiconductor device according to claim 1,wherein the first transistor and the second transistor are N-typetransistors, and wherein the first potential is at a high electricpotential side.
 5. The semiconductor device according to claim 1,wherein the first transistor and the second transistor are P-typetransistors, and wherein the first potential is at a low electricpotential side.
 6. A semiconductor device for boosting or reducing apotential voltage, comprising a first transistor; a second transistor; athird transistor; a first capacitor element; a second capacitor element;and a power supply terminal, wherein one electrode of the firsttransistor is electrically connected to a first wiring, which isconnected to the power supply terminal maintaining a first potential;wherein a gate electrode of the first transistor is electricallyconnected to one electrode of the second transistor and a firstelectrode of the first capacitor element, wherein a gate electrode ofthe second transistor and a first electrode of the second capacitorelement are electrically connected to a second wiring, which is input asecond potential; and wherein a second electrode of the second capacitoris electrically connected to the other electrode of the first transistorand one electrode of the third transistor.
 7. The semiconductor deviceaccording to claim 6, wherein the one electrode of the third transistoris electrically connected to the other electrode of the firsttransistor; and wherein the other electrode of the third transistor iselectrically connected to the other electrode of the second transistor.8. The semiconductor device according to claim 6, wherein the oneelectrode of the third transistor is electrically connected to the otherelectrode of the first transistor and the other electrode of the secondtransistor.
 9. The semiconductor device according to claim 6, whereinthe first transistor and the second transistor are N-type transistors,and wherein the first potential is at a high electric potential side.10. The semiconductor device according to claim 6, wherein the firsttransistor and the second transistor are P-type transistors, and whereinthe first potential is at a low electric potential side.
 11. A DC-DCconverter for boosting or reducing a potential voltage, comprising afirst transistor; a second transistor; a first capacitor element; asecond capacitor element; a rectifying element; and a power supplyterminal, wherein one electrode of the first transistor is electricallyconnected to a first wiring, which is connected to the power supplyterminal maintaining a first potential; wherein a gate electrode of thefirst transistor is electrically connected to one electrode of thesecond transistor and a first electrode of the first capacitor element,wherein a gate electrode of the second transistor and a first electrodeof the second capacitor element are electrically connected to a secondwiring, which is input a second potential; and wherein a secondelectrode of the second capacitor is electrically connected to the otherelectrode of the first transistor and a first electrode of therectifying element.
 12. The DC-DC converter according to claim 11,wherein the first electrode of the rectifying element is electricallyconnected to the other electrode of the first transistor; and wherein asecond electrode of the rectifying element is electrically connected tothe other electrode of the second transistor.
 13. The DC-DC converteraccording to claim 11, wherein the first electrode of the rectifyingelement is electrically connected to the other electrode of the firsttransistor and the other electrode of the second transistor.
 14. TheDC-DC converter according to claim 11, wherein the first transistor andthe second transistor are N-type transistors, and wherein the firstpotential is at a high electric potential side.
 15. The DC-DC converteraccording to claim 11, wherein the first transistor and the secondtransistor are P-type transistors, and wherein the first potential is ata low electric potential side.
 16. A DC-DC converter for boosting orreducing a potential voltage, comprising a first transistor; a secondtransistor; a third transistor; a first capacitor element; a secondcapacitor element; and a power supply terminal, wherein one electrode ofthe first transistor is electrically connected to a first wiring, whichis connected to the power supply terminal maintaining a first potential;wherein a gate electrode of the first transistor is electricallyconnected to one electrode of the second transistor and a firstelectrode of the first capacitor element, wherein a gate electrode ofthe second transistor and a first electrode of the second capacitorelement are electrically connected to a second wiring, which is input asecond potential; and wherein a second electrode of the second capacitoris electrically connected to the other electrode of the first transistorand one electrode of the third transistor.
 17. The DC-DC converteraccording to claim 16, wherein the one electrode of the third transistoris electrically connected to the other electrode of the firsttransistor; and wherein the other electrode of the third transistor iselectrically connected to the other electrode of the second transistor.18. The DC-DC converter according to claim 16, wherein the one electrodeof the third transistor is electrically connected to the other electrodeof the first transistor and the other electrode of the secondtransistor.
 19. The DC-DC converter according to claim 16, wherein thefirst transistor and the second transistor are N-type transistors, andwherein the first potential is at a high electric potential side. 20.The DC-DC converter according to claim 16, wherein the first transistorand the second transistor are P-type transistors, and wherein the firstpotential is at a low electric potential side.